pgsql: Revert "libpqwalreceiver: Convert to libpq-be-fe-helpers.h"

Revert "libpqwalreceiver: Convert to libpq-be-fe-helpers.h"

This reverts commit 728f86fec65537eade8d9e751961782ddb527934.

The signal handling was a few bricks shy of a load in that commit,
which made the walreceiver non-responsive to SIGTERM while it was
waiting for the connection to be established. That prevented a standby
from being promoted.

Since it was non-essential refactoring, let's revert it to make v16
work the same as earlier releases. I reverted it in 'master' too, to
keep the branches in sync. The refactoring was a good idea as such,
but it needs a bit more work. Once we have developed a complete patch
with this issue fixed, let's re-apply that to 'master'.

Reported-by: Kyotaro Horiguchi
Backpatch-through: 16
Discussion: https://www.postgresql.org/message-id/20231231.200741.1078989336605759878.horikyota.ntt@gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c5a6d5337170a133490c91d7f6d27ac25c47480b

Modified Files
--------------
.../libpqwalreceiver/libpqwalreceiver.c | 55 ++++++++++++++++++----
1 file changed, 47 insertions(+), 8 deletions(-)