pgsql: Provide a HINT listing the allowed unit names when a GUC variable

Log Message:
-----------
Provide a HINT listing the allowed unit names when a GUC variable seems to
contain a wrong unit specification, per discussion.
In passing, fix the code to avoid unnecessary integer overflows when
converting units, and to detect overflows when they do occur.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc.c (r1.400 -> r1.401)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.400&r2=1.401)