Add some missing PGDLLIMPORT markings

Add some missing PGDLLIMPORT markings

Three variables in pqsignal.h (UnBlockSig, BlockSig and StartupBlockSig)
were not marked with PGDLLIMPORT, as they are declared in a way that
prevents mark_pgdllimport.pl to detect them.  These variables are
redefined in a style more consistent with the other headers, allowing
the script to find and mark them.

PGDLLIMPORT was missing for __pg_log_level in logging.h, so add it
back.  The marking got accidentally removed in 9a374b77, just after its
addition in 8ec5694.

While on it, add a comment in mark_pgdllimport.pl explaining what are
the arguments needed by the script (aka a list of header paths).

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20220506234924.6mxxotl3xl63db3l@alap3.anarazel.de