pgsql: Provide support for multiplexing SIGUSR1 signal.

Log Message:
-----------
Provide support for multiplexing SIGUSR1 signal. The upcoming synchronous
replication patch needs a signal, but we've already used SIGUSR1 and
SIGUSR2 in normal backends. This patch allows reusing SIGUSR1 for that,
and for other purposes too if the need arises.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        twophase.c (r1.48 -> r1.49)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c?r1=1.48&r2=1.49)
    pgsql/src/backend/commands:
        async.c (r1.142 -> r1.143)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c?r1=1.142&r2=1.143)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.88 -> r1.89)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.88&r2=1.89)
    pgsql/src/backend/storage/ipc:
        sinval.c (r1.86 -> r1.87)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/sinval.c?r1=1.86&r2=1.87)
        sinvaladt.c (r1.74 -> r1.75)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/sinvaladt.c?r1=1.74&r2=1.75)
    pgsql/src/backend/storage/lmgr:
        proc.c (r1.202 -> r1.203)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c?r1=1.202&r2=1.203)
    pgsql/src/backend/tcop:
        postgres.c (r1.558 -> r1.559)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.558&r2=1.559)
    pgsql/src/include/storage:
        proc.h (r1.107 -> r1.108)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h?r1=1.107&r2=1.108)
        sinval.h (r1.48 -> r1.49)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/sinval.h?r1=1.48&r2=1.49)
    pgsql/src/include/tcop:
        tcopprot.h (r1.93 -> r1.94)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/tcopprot.h?r1=1.93&r2=1.94)