POSIX.xs: Never pass NULL to ctermid()

Doing so can cause races.

It is interesting that POSIX:ctermid() takes a parameter, but the
pod doesn't indicate that it does.  Prior to this commit the parameter
was ignored if and only if the platform contains a ctermid_r()
function, and hence on such platforms there was no possibility of a race
here.  The man pages I've seen for ctermid_r() indicate that it differs
from regular ctermid() only in that it will fail if the input is NULL,
and hence a race could occur if it didn't immediately fail.

The situation prior to this commit wa that if you followed the pod on a
non-ctermid_r() platform, and called this without a parameter, it would
call ctermid with NULL, creating a potential race.  This commit changes
so that a race is never possible.

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