Fix processing of PGC_BACKEND GUC parameters on Windows.
Fix processing of PGC_BACKEND GUC parameters on Windows.

EXEC_BACKEND builds (i.e., Windows) failed to absorb values of PGC_BACKEND
parameters if they'd been changed post-startup via the config file.  This
for example prevented log_connections from working if it were turned on
post-startup.  The mechanism for handling this case has always been a bit
of a kluge, and it wasn't revisited when we implemented EXEC_BACKEND.
While in a normal forking environment new backends will inherit the
postmaster's value of such settings, EXEC_BACKEND backends have to read
the settings from the CONFIG_EXEC_PARAMS file, and they were mistakenly
rejecting them.  So this case has always been broken in the Windows port;
so back-patch to all supported branches.

Amit Kapila

jmcatamney/gpdb,50wu/gpdb,ovr/postgres-xl,ashwinstar/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,zeroae/postgres-xl,50wu/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,yazun/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,techdragon/Postgres-XL,pavanvd/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,zeroae/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,zeroae/postgres-xl,adam8157/gpdb,ovr/postgres-xl,jmcatamney/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,oberstet/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,ovr/postgres-xl,lisakowen/gpdb,techdragon/Postgres-XL,oberstet/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,ovr/postgres-xl,oberstet/postgres-xl,yazun/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,yazun/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,ashwinstar/gpdb