pgsql: Fix CREATE INDEX CONCURRENTLY to not deadlock against an

Log Message:
-----------
Fix CREATE INDEX CONCURRENTLY to not deadlock against an automatic or manual
VACUUM that is blocked waiting to get lock on the table being indexed.
Per report and fix suggestion from Greg Stark.

Modified Files:
--------------
    pgsql/src/backend/commands:
        indexcmds.c (r1.169 -> r1.170)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/indexcmds.c?r1=1.169&r2=1.170)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.39&r2=1.40)
    pgsql/src/include/storage:
        procarray.h (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h?r1=1.19&r2=1.20)