RT #76248: double-freed SV with nested sig-handler

RT #76248: double-freed SV with nested sig-handler

There was some buggy code in Perl_sighandler() related to getting an SV
with the signal name to pass to the perl-level handler function.
`
Basically:

on threaded builds, a sig handler that died leaked PL_psig_name[sig];
on unthreaded builds, in a recursive handler that died, PL_sig_sv was
prematurely freed.

PL_sig_sv was originally just a file static var that was not
recursion-save anyway, and got promoted to perlvars.h when it should
instead have been done away with. So I've got rid of it now, and
rationalised the code, which fixed the two issues listed above.

Also added an assert which makes the dodgy manual popping of the save
stack slightly less dodgy.