pgsql: Add helper library for use of libpq inside the server environmen

Add helper library for use of libpq inside the server environment

Currently dblink and postgres_fdw don't process interrupts during connection
establishment. Besides preventing query cancellations etc, this can lead to
undetected deadlocks, as global barriers are not processed.

Libpqwalreceiver in contrast, processes interrupts during connection
establishment. The required code is not trivial, so duplicating it into
additional places does not seem like a good option.

These aforementioned undetected deadlocks are the reason for the spate of CI
test failures in the FreeBSD 'test_running' step.

For now the helper library is just a header, as it needs to be linked into
each extension using libpq, and it seems too small to be worth adding a
dedicated static library for.

The conversion to the helper are done in subsequent commits.

Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://postgr.es/m/20220925232237.p6uskba2dw6fnwj2@awork3.anarazel.de

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9e129a2243e2c1e5b7107cec06262aad26533f0c
Author: Andres Freund <andres(at)anarazel(dot)de>

Modified Files
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src/include/libpq/libpq-be-fe-helpers.h | 242 ++++++++++++++++++++++++++++++++
1 file changed, 242 insertions(+)