pgsql: Arrange for autovacuum to be killed when another operation wants

Log Message:
-----------
Arrange for autovacuum to be killed when another operation wants to be alone
accessing it, like DROP DATABASE.  This allows the regression tests to pass
with autovacuum enabled, which open the gates for finally enabling autovacuum
by default.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        twophase.c (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.26&r2=1.27)
    pgsql/src/backend/commands:
        dbcommands.c (r1.188 -> r1.189)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.188&r2=1.189)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.30&r2=1.31)
        postmaster.c (r1.507 -> r1.508)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.507&r2=1.508)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.20 -> r1.21)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c.diff?r1=1.20&r2=1.21)
    pgsql/src/backend/storage/lmgr:
        proc.c (r1.182 -> r1.183)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.182&r2=1.183)
    pgsql/src/include/postmaster:
        autovacuum.h (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/autovacuum.h.diff?r1=1.6&r2=1.7)
    pgsql/src/include/storage:
        proc.h (r1.92 -> r1.93)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h.diff?r1=1.92&r2=1.93)
        procarray.h (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h.diff?r1=1.11&r2=1.12)