pgsql: Don't use BLCKSZ for the physical length of the pg_control file,

Log Message:
-----------
Don't use BLCKSZ for the physical length of the pg_control file, but
instead a dedicated symbol.  This probably makes no functional difference
for likely values of BLCKSZ, but it makes the intent clearer.
Simon Riggs, minor editorialization by Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.232 -> r1.233)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.232&r2=1.233)
    pgsql/src/bin/pg_resetxlog:
        pg_resetxlog.c (r1.41 -> r1.42)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_resetxlog/pg_resetxlog.c.diff?r1=1.41&r2=1.42)
    pgsql/src/include/catalog:
        pg_control.h (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_control.h.diff?r1=1.28&r2=1.29)