Log Message:
emulators/simh: update to snapshot dated 2021-10-12.

Since there are no simh releases there are also no release notes to summarize.
The previous snapshot was over 3 years ago: 2018-09-19.

A somewhat controversial change with some people is that simh now
appends a "footer" block to disk image files, containing metadata
(struct simh_disk_footer).  ATTACH -r <disk_unit> <filename> will avoid
this but makes the device read-only.

New information from the README.md file:

    DO <stdin>                  Invokes a nested DO command with input from the
                                running console.
    RUNLIMIT			Bound simulator execution time
    TAR                         Manipulate file archives
    CURL                        Access URLs from the web

Changes in the built emulator models:
- several new VAX models (vax8200, vaxstation{3100,4000}*, infoserver*)
- several new PDP-10 models (pdp10-{ka,ki,kl})
- new: pdp6
- Intel-Systems: Merge MDS, SDK, OEM simulators into Intel-MDS simulator

The files ka655x.bin and vmb.exe are no longer installed since they are
not required for operation (they are built into the VAX models that use
them), and it is strange to single out these ROM images out of over 30 to
install.  LOAD -r can load alternative versions.