maint: tighten strncmp syntax check

maint: tighten strncmp syntax check

Using 'int ret = strcmp(a, b)' in a qsort function is a valid use of
str[n]cmp that should _not_ be turned to STREQ, but it was falling
foul of our specific syntax-check.  Meanwhile, gnulib's maint.mk
already has a tighter bound for strcmp, so we can copy that regex and
just check for strncmp, which results in fewer false positives that
require exceptions.

* cfg.mk (sc_prohibit_strcmp_and_strncmp): Rename...
(sc_prohibit_strncmp): ...to this, and tighten, to mirror
maint.mk's sc_prohibit_strcmp's better regex.
* Makefile.am (syntax_check_exceptions): Update exception rule.
* .x-sc_prohibit_strcmp_and_strncmp: Rename...
* .x-sc_prohibit_strncmp: ...and trim.