pgsql: Limit NBuffers and some related values to INT_MAX / BLCKSZ, to

Log Message:
-----------
Limit NBuffers and some related values to INT_MAX / BLCKSZ, to prevent
arithmetic overflow during initial sizing calculations.  This is not
water-tight but it should avoid the grossest sorts of failures.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc.c (r1.243 -> r1.244)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.243&r2=1.244)