pgsql: On platforms that have getrlimit(RLIMIT_STACK), use it to ensure

Log Message:
-----------
On platforms that have getrlimit(RLIMIT_STACK), use it to ensure that
max_stack_depth is not set to an unsafe value.

This commit also provides configure-time checking for <sys/resource.h>,
and cleans up some perhaps-unportable code associated with use of that
include file and getrlimit().

Modified Files:
--------------
    pgsql:
        configure (r1.511 -> r1.512)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.511&r2=1.512)
        configure.in (r1.481 -> r1.482)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.481&r2=1.482)
    pgsql/contrib/pgbench:
        pgbench.c (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgbench/pgbench.c.diff?r1=1.56&r2=1.57)
    pgsql/doc/src/sgml:
        config.sgml (r1.89 -> r1.90)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.89&r2=1.90)
    pgsql/src/backend/tcop:
        postgres.c (r1.511 -> r1.512)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.511&r2=1.512)
    pgsql/src/backend/utils/misc:
        guc.c (r1.355 -> r1.356)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.355&r2=1.356)
    pgsql/src/include:
        pg_config.h.in (r1.104 -> r1.105)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.104&r2=1.105)
    pgsql/src/include/tcop:
        tcopprot.h (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/tcopprot.h.diff?r1=1.84&r2=1.85)
    pgsql/src/include/utils:
        pg_rusage.h (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/pg_rusage.h.diff?r1=1.2&r2=1.3)