maint: move coreutils-specific syntax check rules into cfg.mk

maint: move coreutils-specific syntax check rules into cfg.mk

* maint.mk (sc_dd_max_sym_length, sc_prohibit_jm_in_m4):
(sc_root_tests, sc_always_defined_macros, sc_system_h_headers):
(sc_sun_os_names, sc_tight_scope, sc_no_exec_perl_coreutils):
Move rules into cfg.mk.
(sc_strftime_check): Renamed from strftime-check, and moved, too.
* cfg.mk: Add the above rules.