pgsql: Improve pg_regress.c's infrastructure for issuing psql commands.

Improve pg_regress.c's infrastructure for issuing psql commands.

Support issuing more than one "-c command" switch to a single
psql invocation.  This allows combining some things that formerly
required two or more backend launches into a single session.
In particular, we can issue DROP DATABASE as one of the -c commands
without getting "DROP DATABASE cannot run inside a transaction block".

In addition to reducing the number of sessions needed, this patch
also suppresses "NOTICE:  database "foo" does not exist, skipping"
chatter that was formerly generated during pg_regress's DROP DATABASE
(or ROLE) IF NOT EXISTS calls.  That moves us another step closer
to the ideal of not seeing any messages during successful build/test.

This also eliminates some hard-coded restrictions on the length of
the commands issued.  I don't think we were anywhere near hitting
those, but getting rid of the limit is comforting.

Patch by me, but thanks to Nathan Bossart for starting the discussion.

Discussion: https://postgr.es/m/DCBAE0E4-BD56-482F-8A70-7FD0DC0860BE@amazon.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f45dc59a38cab1d2af6baaedb79559fe2e9b3781

Modified Files
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src/test/regress/pg_regress.c | 148 +++++++++++++++++++++++++++++-------------
1 file changed, 104 insertions(+), 44 deletions(-)