core: prefix selinux messages with "selinux: "

SELinux outputs semi-random messages like "Unknown permission start for class
system", and the user has to dig into message metadata to find out where
they are comming from. Add a prefix to give a hint.

src/core/selinux-access.c

@@ -110,6 +110,7 @@ static int callback_type_to_priority(int type) {
 */
 _printf_(2, 3) static int log_callback(int type, const char *fmt, ...) {
         va_list ap;
+        const char *fmt2;
 
 #ifdef HAVE_AUDIT
         int fd;
@@ -131,8 +132,10 @@ _printf_(2, 3) static int log_callback(int type, const char *fmt, ...) {
         }
 #endif
 
+        fmt2 = strjoina("selinux: ", fmt);
+
         va_start(ap, fmt);
-        log_internalv(LOG_AUTH | callback_type_to_priority(type), 0, __FILE__, __LINE__, __FUNCTION__, fmt, ap);
+        log_internalv(LOG_AUTH | callback_type_to_priority(type), 0, __FILE__, __LINE__, __FUNCTION__, fmt2, ap);
         va_end(ap);
 
         return 0;