maint.mk: allow fine-grained syntax-check exclusion via Make variables

maint.mk: allow fine-grained syntax-check exclusion via Make variables

Before, you would have had to create one .x-sc_ file per rule in order
to exempt offending files.  Now, you may instead use a Make variable --
usually defined in cfg.mk -- whose name identifies the affected rule.
* top/maint.mk (_sc_excl): Define.
(VC_LIST_EXCEPT): Use it to exclude names on a per-rule basis.
(_sc_search_regexp): When not using VC_LIST_EXCEPT, exclude here, too.