build-sys: use -Og instead of -O0 to catch warnings

$ touch src/core/dbus.c; make CFLAGS=-O0
  make --no-print-directory all-recursive
  Making all in .
    CC       src/core/libsystemd_core_la-dbus.lo
    CCLD     libsystemd-core.la

  $ touch src/core/dbus.c; make CFLAGS=-Og
  make --no-print-directory all-recursive
  Making all in .
    CC       src/core/libsystemd_core_la-dbus.lo
  src/core/dbus.c: In function 'init_registered_system_bus':
  src/core/dbus.c:798:18: warning: 'id' may be used uninitialized in this function [-Wmaybe-uninitialized]
           dbus_free(id);
                    ^
    CCLD     libsystemd-core.la

-Og Optimize debugging experience. -Og enables optimizations that do
not interfere with debugging. It should be the optimization level of
choice for the standard edit-compile-debug cycle, offering a
reasonable level of optimization while maintaining fast compilation
and a good debugging experience.

src/shared/ask-password-api.c

@@ -250,7 +250,8 @@ static int create_socket(char **name) {
         } sa = {
                 .un.sun_family = AF_UNIX,
         };
-        int one = 1, r;
+        int one = 1;
+        int r = 0;
         char *c;
 
         assert(name);