pgsql: Put back pqsignal() as an exported libpq symbol.

Put back pqsignal() as an exported libpq symbol.

This reverts commit f7ab80285. Per discussion, we can't remove an
exported symbol without a SONAME bump, which we don't want to do.
In particular that breaks usage of current libpq.so with pre-9.3
versions of psql etc, which need libpq to export pqsignal().

As noted in that commit message, exporting the symbol from libpgport.a
won't work reliably; but actually we don't want to export src/port's
implementation anyway. Any pre-9.3 client is going to be expecting the
definition that pqsignal() had before 9.3, which was that it didn't
set SA_RESTART for SIGALRM. Hence, put back pqsignal() in a separate
source file in src/interfaces/libpq, and give it the old semantics.

Back-patch to v12.

Discussion: https://postgr.es/m/E1g5vmT-0003K1-6S@gemulon.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7ed1bcaed6ecbb0760169ea193cbe70071012576

Modified Files
--------------
src/interfaces/libpq/Makefile | 2 +-
src/interfaces/libpq/exports.txt | 2 +-
src/interfaces/libpq/legacy-pqsignal.c | 57 ++++++++++++++++++++++++++++++++++
3 files changed, 59 insertions(+), 2 deletions(-)