Remove pqsignal() from libpq's official exports list.

Client applications should get this function, if they need it, from
libpgport.

The fact that it's exported from libpq is a hack left over from before
we set up libpgport.  It's never been documented, and there's no good
reason for non-PG code to be calling it anyway, so hopefully this won't
cause any problems.  Moreover, with the previous setup it was not real
clear whether our clients that use the function were getting it from
libpgport or libpq, so this might actually prevent problems.

The reason for changing it now is that in the wake of commit ea53100d5,
some linkers won't export the symbol, apparently because it's coming from
a .a library instead of a .o file.  We could get around that by continuing
to symlink pqsignal.c into libpq as before; but unless somebody complains
very hard, I don't want to adopt such a kluge.

Discussion: https://postgr.es/m/13022.1538003440@sss.pgh.pa.us
Discussion: https://postgr.es/m/E1g5Y8r-0006vs-QA@gemulon.postgresql.org

src/interfaces/libpq/exports.txt

@@ -116,7 +116,7 @@ PQserverVersion           113
 PQgetssl                  114
 pg_char_to_encoding       115
 pg_valid_server_encoding  116
-pqsignal                  117
+# pqsignal                117  # no longer exported, see libpgport instead
 PQprepare                 118
 PQsendPrepare             119
 PQgetCancel               120