Revert SIGUSR1 multiplexing patch, per Tom's objection.

src/backend/access/transam/twophase.c

src/backend/commands/async.c

src/backend/postmaster/autovacuum.c

src/backend/storage/ipc/sinval.c

src/backend/storage/ipc/sinvaladt.c

src/backend/storage/lmgr/proc.c

src/backend/tcop/postgres.c

src/include/storage/proc.h

src/include/storage/sinval.h

src/include/tcop/tcopprot.h