pgsql: Add wrapper function libpqrcv_PQexec() in the walreceiver that

Log Message:
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Add wrapper function libpqrcv_PQexec() in the walreceiver that uses async
libpq to send queries, making the waiting for responses interruptible on
platforms where PQexec() can't normally be interrupted by signals, such
as win32.

Fujii Masao and Magnus Hagander

Modified Files:
--------------
pgsql/src/backend/replication:
walreceiver.c (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walreceiver.c?r1=1.8&r2=1.9)
pgsql/src/backend/replication/libpqwalreceiver:
libpqwalreceiver.c (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c?r1=1.8&r2=1.9)