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.

.x-sc_prohibit_strcmp_and_strncmp

-^gnulib/
-^ChangeLog-old$
-^docs/
-^examples/domain-events/events-c/event-test\.c$
-^src/internal\.h$
-^src/lxc/lxc_container\.c$
-^src/node_device/node_device_devkit\.c$
-^src/node_device/node_device_hal\.c$
-^src/storage/parthelper\.c$

.x-sc_prohibit_strncmp

+^src/internal\.h$

Makefile.am

@@ -35,7 +35,7 @@ EXTRA_DIST = \
   .x-sc_prohibit_nonreentrant \
   .x-sc_prohibit_sprintf \
   .x-sc_prohibit_strcmp \
-  .x-sc_prohibit_strcmp_and_strncmp \
+  .x-sc_prohibit_strncmp \
   .x-sc_prohibit_strncpy \
   .x-sc_prohibit_test_minus_ao \
   .x-sc_prohibit_VIR_ERR_NO_MEMORY \

cfg.mk

@@ -231,12 +231,14 @@ sc_avoid_write:
 	halt='consider using safewrite instead of write'		\
 	  $(_sc_search_regexp)
 
-# Use STREQ rather than comparing strcmp == 0, or != 0.
-# Similarly, use STREQLEN or STRPREFIX rather than strncmp.
-sc_prohibit_strcmp_and_strncmp:
-	@prohibit='strn?cmp *\('					\
-	halt='use STREQ() in place of the above uses of str[n]cmp'	\
-	  $(_sc_search_regexp)
+# Similar to the gnulib maint.mk rule for sc_prohibit_strcmp
+# Use STREQLEN or STRPREFIX rather than comparing strncmp == 0, or != 0.
+sc_prohibit_strncmp:
+	@grep -nE '! *str''ncmp *\(|\<str''ncmp *\([^)]+\) *=='		\
+	    $$($(VC_LIST_EXCEPT))					\
+	  | grep -vE ':# *define STREQ\(' &&				\
+	  { echo '$(ME): use STREQLEN or STRPREFIX instead of str''ncmp' \
+		1>&2; exit 1; } || :
 
 # Use virAsprintf rather than as'printf since *strp is undefined on error.
 sc_prohibit_asprintf: