Quick exit on log stream child exit in pg_basebackup

Quick exit on log stream child exit in pg_basebackup

If the log streaming child process (thread on Windows) dies during
backup then the whole backup will be aborted at the end of the
backup.  Instead, trap ungraceful termination of the log streaming
child and exit early.  This also adds a TAP test for simulating this
by terminating the responsible backend.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/0F69E282-97F9-4DB7-8D6D-F927AA6340C8@yesql.se
Discussion: https://postgr.es/m/VI1PR83MB0189818B82C19059CB62E26199A89@VI1PR83MB0189.EURPRD83.prod.outlook.com