Log Message:
Update to 3.28.1

* Bump nspr requirement

Changelog:
3.28.1:
The NSS team has released Network Security Services (NSS) 3.28.1,
which is a patch release.

Below is a summary of the changes.
Please refer to the full release notes for additional details,
including the SHA256 fingerprints of the changed CA certificates.

No new functionality is introduced in this release. This is a patch release to
update the list of root CA certificates and address a minor TLS compatibility
issue that some applications experienced with NSS 3.28.

Notable Changes:
* The following CA certificates were Removed
- CN = Buypass Class 2 CA 1
- CN = Root CA Generalitat Valenciana
- OU = RSA Security 2048 V3
* The following CA certificates were Added
- OU = AC RAIZ FNMT-RCM
- CN = Amazon Root CA 1
- CN = Amazon Root CA 2
- CN = Amazon Root CA 3
- CN = Amazon Root CA 4
- CN = LuxTrust Global Root 2
- CN = Symantec Class 1 Public Primary Certification Authority - G4
- CN = Symantec Class 1 Public Primary Certification Authority - G6
- CN = Symantec Class 2 Public Primary Certification Authority - G4
- CN = Symantec Class 2 Public Primary Certification Authority - G6
* The version number of the updated root CA list has been set to 2.11
* A misleading assertion/alert has been removed when NSS tries to flush data
  to the peer but the connection was already reset.

3.28:
The NSS team has released Network Security Services (NSS) 3.28,
which is a minor release.

Below is a summary of the changes.

Please refer to the full release notes for additional details:
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/NSS_3.28_release_notes

Request to test and prepare for TLS 1.3 (draft):
================================================
To prepare for a change of default build options, which is
planned for
the future NSS 3.29 release, we'd like to encourage all users of NSS
3.28
to override the standard NSS build configuration to enable support for
(draft
) TLS 1.3 by defining NSS_ENABLE_TLS_1_3=1 at build time.
We'd like to ask you to
please give feedback to the NSS developers for any
compatibility issues that you
might encounter in your tests.

For providing feedback, you may send a message to this mailing list, see:
  https://lists.mozilla.org/listinfo/dev-tech-crypto
or please report a bug here:
  https://bugzilla.mozilla.org/enter_bug.cgi?product=NSS

New functionality:
==================
* NSS includes support for TLS 1.3 draft -18. This includes a number 
  of
improvements to TLS 1.3:
  - The signed certificate timestamp, used in
certificate transparency, 
    is supported in TLS 1.3.
  - Key exporters for TLS
1.3 are supported. This includes the early key
    exporter, which can be used if
0-RTT is enabled. Note that there is a
    difference between TLS 1.3 and key
exporters in older versions of TLS.
    TLS 1.3 does not distinguish between an
empty context and no context.
  - The TLS 1.3 (draft) protocol can be enabled, by
defining
    NSS_ENABLE_TLS_1_3=1 when building NSS.
* NSS includes support for
the X25519 key exchange algorithm, which is
  supported and enabled by default in
all versions of TLS.

New Functions:
==============
* SSL_ExportEarlyKeyingMaterial
* SSL_SendAdditionalKeyShares
* SSL_SignatureSchemePrefSet
* SSL_SignatureSchemePrefGet

Notable Changes:
================
* NSS can no longer be compiled with support for additional elliptic curves.
  This was previously possible by replacing certain NSS source files.
* NSS will now detect the presence of tokens that support additional
  elliptic curves and enable those curves for use in TLS.
  Note that this detection has a one-off performance cost, which can be
  avoided by using the SSL_NamedGroupConfig function to limit supported
  groups to those that NSS provides.
* PKCS#11 bypass for TLS is no longer supported and has been removed.
* Support for "export" grade SSL/TLS cipher suites has been removed.
* NSS now uses the signature schemes definition in TLS 1.3.
  This also affects TLS 1.2. NSS will now only generate signatures with the
  combinations of hash and signature scheme that are defined in TLS 1.3,
  even when negotiating TLS 1.2.
  - This means that SHA-256 will only be used with P-256 ECDSA certificates,
    SHA-384 with P-384 certificates, and SHA-512 with P-521 certificates.
    SHA-1 is permitted (in TLS 1.2 only) with any certificate for backward
    compatibility reasons.
  - New functions to configure signature schemes are provided:
    SSL_SignatureSchemePrefSet, SSL_SignatureSchemePrefGet.
    The old SSL_SignaturePrefSet and SSL_SignaturePrefSet functions are
    now deprecated.
  - NSS will now no longer assume that default signature schemes are 
    supported by a peer if there was no commonly supported signature scheme.
* NSS will now check if RSA-PSS signing is supported by the token that holds
  the private key prior to using it for TLS.
* The certificate validation code contains checks to no longer trust
  certificates that are issued by old WoSign and StartCom CAs after 
  October 21, 2016. This is equivalent to the behavior that Mozilla will
  release with Firefox 51.