Remove thread context from Perl_vmssetuserlnm.

Remove thread context from Perl_vmssetuserlnm.

This routine by its very nature applies to the whole process so
there is no way it can make use of a thread context, and it may need
to be called from places where there is no thread context, such
as very early in start-up.

It's not documented, was never intended to be part of the API, was
only made global so it could be called from doio.c, and no uses of
it turn up in a CPAN grep, so the change should be safe.