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.c

@@ -115,17 +115,23 @@ size_t page_size(void) {
         return pgsz;
 }
 
-bool streq_ptr(const char *a, const char *b) {
-
-        /* Like streq(), but tries to make sense of NULL pointers */
+int strcmp_ptr(const char *a, const char *b) {
 
+        /* Like strcmp(), but tries to make sense of NULL pointers */
         if (a && b)
-                return streq(a, b);
+                return strcmp(a, b);
 
-        if (!a && !b)
-                return true;
+        if (!a && b)
+                return -1;
 
-        return false;
+        if (a && !b)
+                return 1;
+
+        return 0;
+}
+
+bool streq_ptr(const char *a, const char *b) {
+        return strcmp_ptr(a, b) == 0;
 }
 
 char* endswith(const char *s, const char *postfix) {

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)))