string-util: add strstrafter()

strstrafter() is like strstr() but returns a pointer to the first
character *after* the found substring, not on the substring itself.
Quite often this is what we actually want.

Inspired by #27267 I think it makes sense to add a helper for this,
to avoid the potentially fragile manual pointer increment afterwards.

src/shared/edit-util.c

@@ -300,10 +300,8 @@ static int strip_edit_temp_file(EditFile *e) {
 
                 assert(e->context->marker_end);
 
-                contents_start = strstr(old_contents, e->context->marker_start);
-                if (contents_start)
-                        contents_start += strlen(e->context->marker_start);
-                else
+                contents_start = strstrafter(old_contents, e->context->marker_start);
+                if (!contents_start)
                         contents_start = old_contents;
 
                 contents_end = strstr(contents_start, e->context->marker_end);