Fix a few crashes and leaks when cloning C strings.

Fix a few crashes and leaks when cloning C strings.

* alloc.c, lisp.h (dupstring): New function.
* gtkutil.c (xg_get_font):
* term.c (tty_default_color_capabilities):
* xsettings.c (store_monospaced_changed)
(store_font_name_changed, parse_settings)
(read_and_apply_settings, init_gsettings, init_gconf): Use it.
This avoids some unlikely crashes due to accessing freed storage,
and avoids some minor memory leaks in the more-typical case.