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)