pgsql: Suppress log spam from multiple reports of SIGQUIT shutdown.

Suppress log spam from multiple reports of SIGQUIT shutdown.

When the postmaster sends SIGQUIT to its children, there's no real
need for all the children to log that fact; the postmaster already
made a log entry about it, so adding perhaps dozens or hundreds of
child-process log entries adds nothing of value.  So, let's introduce
a new ereport level to specify "WARNING, but never send to log" and
use that for these messages.

Such a change wouldn't have been desirable before commit 7e784d1dc,
because if someone manually SIGQUIT's a backend, we *do* want to log
that.  But now we can tell the difference between a signal that was
issued by the postmaster and one that was not with reasonable
certainty.

While we're here, also clear error_context_stack before ereport'ing,
to prevent error callbacks from being invoked in the signal-handler
context.  This should reduce the odds of getting hung up while trying
to notify the client.

Per a suggestion from Andres Freund.

Discussion: https://postgr.es/m/20201225230331.hru3u6obyy6j53tk@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1f9158ba48122fa232db955a2ee324eec1848ba9

Modified Files
--------------
src/backend/tcop/postgres.c    | 16 ++++++++++++++--
src/backend/utils/error/elog.c | 10 +++++++++-
src/include/utils/elog.h       | 12 ++++++------
3 files changed, 29 insertions(+), 9 deletions(-)