pgsql: Add LOG_NEVER error level code.

Add LOG_NEVER error level code.

This logging level means not to emit the log, which is useful for
functions like relation_needs_vacanalyze().  This function accepts
a log level argument but not all callers want it to emit logs.

Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3101163.1775676098%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/60165db6e1f21ec6cbfce4fd8fd1e521a043d954

Modified Files
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src/backend/postmaster/autovacuum.c | 3 ++-
src/include/utils/elog.h            | 1 +
2 files changed, 3 insertions(+), 1 deletion(-)