pg_regress: Promptly detect failed postmaster startup.
pg_regress: Promptly detect failed postmaster startup.

Detect it the way pg_ctl's wait_for_postmaster() does.  When pg_regress
spawned a postmaster that failed startup, we were detecting that only
with "pg_regress: postmaster did not respond within 60 seconds".
Back-patch to 9.4 (all supported versions).

Reviewed by Tom Lane.

Discussion: 7a2f345d0fc2c30da2feaba67e171f60a20a6e82@gust.leadboat.com

greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb