Revert "Restore errno after VMS hack in S_sv_gets_read_record."

This reverts commit d46f021e36854e800770363f716e7b4a846102ef.

This can be done more universally (and from the point of view of
sv.c, less obtrusively) in Perl_flex_fstat in vms/vms.c.
Make vms.c's Perl_flex_fstat preserve errno on success.

The CRTL's fstat() sets errno to EVMSERR and vaxc$errno to RMS$_IOP
when called on a proccess-permanent file (i.e., stdin, stdout, or
stderr).  That error generally means a rewind operation on a file
that cannot be rewound.  It's odd that fstat is doing such a thing,
but we can at least protect ourselves from the effects of it by
saving errno and restoring it on a successful call.

This cures a couple of test failures and TODOs in t/io/errno.t.
Copyright update for vms/vms.c.

Happy 20th Anniversary, Charles.