shared: add nm_ref_string_equals_str() helper

shared/nm-glib-aux/nm-ref-string.h

@@ -49,6 +49,18 @@ nm_ref_string_get_len (NMRefString *rstr)
 	return rstr ? rstr->len : 0u;
 }
 
+static inline gboolean
+nm_ref_string_equals_str (NMRefString *rstr, const char *s)
+{
+	/* Note that rstr->len might be greater than strlen(rstr->str). This function does
+	 * not cover that and would ignore everything after the first NUL byte. If you need
+	 * that distinction, this function is not for you. */
+
+	return   rstr
+	       ? nm_streq (rstr->str, s)
+	       : (s == NULL);
+}
+
 static inline gboolean
 NM_IS_REF_STRING (const NMRefString *rstr)
 {