Improve static checking when configured --with-ns.

Improve static checking when configured --with-ns.

See Samuel Bronson's remarks in
<http://lists.gnu.org/archive/html/emacs-devel/2012-07/msg00146.html>.
* configure.in (WARN_CFLAGS): Omit -Wunreachable-code, as it's
a no-op with recent GCC and harmful in earlier ones.
Omit -Wsync-nand, as it's irrelevant to Emacs and provokes a
warning when compiling with ObjC.  Always omit
-Wunsafe-loop-optimizations, as we don't mind when optimization is
being done correctly.

Fix some minor --with-ns problems found by static checking.
* src/frame.c (Ftool_bar_pixel_width) [!FRAME_TOOLBAR_WIDTH]:
(x_set_font) [!HAVE_X_WINDOWS]:
* src/image.c (xpm_load_image) [HAVE_NS]:
(x_to_xcolors) [!HAVE_X_WINDOWS && !HAVE_NTGUI]:
(x_disable_image) [!HAVE_NS && !HAVE_NTGUI]:
Remove unused local.
(Fx_parse_geometry) [HAVE_NS]: Don't return garbage.
(xpm_load_image) [HAVE_NS && !HAVE_XPM]: Remove unused label.
* src/image.c (x_create_bitmap_from_file) [HAVE_NS]:
(xpm_load_image, xpm_load) [HAVE_NS && !HAVE_XPM]:
* src/nsselect.m (symbol_to_nsstring, ns_string_to_pasteboard_internal):
* src/xfaces.c (Fx_load_color_file) [!HAVE_X_WINDOWS]:
Fix pointer signedness problem.
* src/xfaces.c (FRAME_X_FONT_TABLE):
* src/xterm.h (FRAME_X_FONT_TABLE): Remove unused, incompatible macros.