pgsql: Arrange for SIGINT in autovacuum workers to cancel the current

Log Message:
-----------
Arrange for SIGINT in autovacuum workers to cancel the current table and
continue with the schedule.  Change current uses of SIGINT to abort a worker
into SIGTERM, which keeps the old behaviour of terminating the process.

Patch from ITAGAKI Takahiro, with some editorializing of my own.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.51&r2=1.52)
        postmaster.c (r1.528 -> r1.529)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.528&r2=1.529)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c.diff?r1=1.26&r2=1.27)
    pgsql/src/backend/tcop:
        postgres.c (r1.534 -> r1.535)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.534&r2=1.535)