src/*.c: Silence a few warnings about unused vars and functions.

src/*.c: Silence a few warnings about unused vars and functions.

* src/image.c (x_bitmap_height, x_bitmap_width) [HAVE_X_WINDOWS]:
* src/sysdep.c (reset_sigio) [!DOS_NT]: Declare conditionally.

* src/keyboard.c (read_decoded_event_from_main_queue): #ifdef out
  variables on Windows.

* src/w32.c (unsetenv): Remove unused var `retval'.
  (emacs_gnutls_pull): Remove unused vars `fdset' and `timeout'.

* src/w32fns.c (Ffile_system_info): Use parenthesis in and/or expression.

* src/w32notify.c (watch_worker): Remove unnecesary var sleep_result.
  (start_watching): Remove unused var `thr'.

* src/w32proc.c (sys_spawnve): Comment out unused vars `first', `last'.
  (find_child_console): Remove unnecesary var `thread_id'.

* src/w32term.c (w32_read_socket): Comment out unused vars `row', `columns'.
  (x_focus_frame): #ifdef 0 unused variable `dpyinfo'.