maint: ensure dd's computation of O_FULLBLOCK uses all O_ symbol names

maint: ensure dd's computation of O_FULLBLOCK uses all O_ symbol names

* src/dd.c (O_FULLBLOCK): Use a more uniform initializer, that makes
it easier to extract all O_ symbol names.
* maint.mk (syntax-check-rules): Also search for sc_ rules in cfg.mk.
(sc_root_tests): Ensure that this rule sets $diff.
* cfg.mk (sc_dd_O_FLAGS): New rule.