unix: Fix SIGINFO and SIGWINCH.

It seems these two signals have not been working since at least
2009. Detection of these signals seems to have never been added to
the configure.ac script and the code guarded by #ifdef then bit-rotted
(the idiom used to handle these signals seems to have been abandoned
for something simpler/better in 2009). This fix simply handles these
signals the same way the other signals are handled in
System/Posix/Signals.hsc.

Closes: https://github.com/haskell/unix/issues/30