busctl: add and use strcmp_ptr()

In member_compare_func(), it compares interface, type and name of
members.  But as it can contain NULL pointer, it needs to check them
before calling strcmp().  So make it as a separate strcmp_ptr
function (named after streq_ptr) so that it can be used by others.

Also let streq_ptr() to use it in order to make the code simpler.

src/basic/util.h

@@ -71,6 +71,7 @@ size_t page_size(void) _pure_;
 #define strncaseeq(a, b, n) (strncasecmp((a), (b), (n)) == 0)
 
 bool streq_ptr(const char *a, const char *b) _pure_;
+int strcmp_ptr(const char *a, const char *b) _pure_;
 
 #define new(t, n) ((t*) malloc_multiply(sizeof(t), (n)))