If the first argument of sigaction() was a string, not a number

If the first argument of sigaction() was a string, not a number
(or a SIGXXX 'constant') one got first (if using -w) 'Argument "FOO"
isn't numeric in subroutine entry ...' but after that one got
(depending on the OS) either a coredump (because of trying to
assign to *0 in mg_get) or a hang (because of the sigprocmask()
blocking signals inside POSIX::sigaction, a nasty hang since
one obviously cannot interrupt it...only SIGKILL works).
In older Perls (tried with 5.6.1) one got 'No such signal: SIGZERO ...'
because of the string becoming zero due to the XS typemap magic.
Resolved by making the POSIX::sigaction to try harder to figure
out a valid signal number (one still gets the warning, though),
and returning undef if no sense can be made.

p4raw-id: //depot/perl@19809