Avoid calling pqsignal() with invalid signals on Windows frontends.

Avoid calling pqsignal() with invalid signals on Windows frontends.

As noted by the comment at the top of port/pqsignal.c, Windows
frontend programs can only use pqsignal() with the 6 signals
required by C.  Most places avoid using invalid signals via #ifndef
WIN32, but initdb and pg_test_fsync check whether the signal itself
is defined, which doesn't work because win32_port.h defines many
extra signals for the signal emulation code.  pg_regress seems to
have missed the memo completely.  These issues aren't causing any
real problems today because nobody checks the return value of
pqsignal(), but a follow-up commit will add some error checking.

To fix, surround all frontend calls to pqsignal() that use signals
that are invalid on Windows with #ifndef WIN32.  We cannot simply
skip defining the extra signals in win32_port.h for frontends
because they are needed in places such as pgkill().

Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/Z4chOKfnthRH71mw%40nathan