VMS posix exit fixes

VMS posix exit fixes

perl.h and perl.c need further fixes to get VMS to return the expected
POSIX exit codes when that is enabled.

This fix gets the correct numbers except for the SIGTERM case, which
will need some more work.

It also gets the posix exit code to set an error severity on a fatal
exit so that DCL and MMS/MMK or VMS native programs can easily detect a
script failure.

This patch does not address an issue in vms.c where the feature logicals
may not be correctly read.  That will follow in a future patch.

The tests have been adjusted to detect when VMS is in the POSIX exit
mode and perform properly.

-John
wb8tyw@gmail.com
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