Log Message:
* Rename the buildlink module name associated with the security/PAM
  package from "pam" to "linux-pam".

* Rewrite PAM/builtin.mk to check that we have Linux-PAM, and re-classify
  MacOS X's PAM as Linux-PAM because it _is_, according to to Apple.
  Also don't use BUILDLINK_TRANSFORM.* to rewrite header file paths
  -- just use a symlink so that <security/*.h> can be used to find
  <pam/*.h>.