internal: Introduce STRCASESKIP()

There is so far one case where STRCASEPREFIX(a, b) && a +
strlen(b) combo is used (in virVMXConfigScanResultsCollector()),
but there will be more. Do what we do usually: introduce a macro.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>

docs/coding-style.rst

@@ -750,6 +750,15 @@ use one of the following semantically named macros
       * use: */
      STRSKIP(a, b)
 
+-  For skipping prefix case insensitively:
+
+   ::
+
+     /* Instead of:
+      *   STRCASEPREFIX(a, b) ? a + strlen(b) : NULL
+      * use: */
+     STRCASESKIP(a, b)
+
 -  To avoid having to check if a or b are NULL:
 
    ::