Fix -Wwrite_strings in general and for Gtk+ specific code.

Fix -Wwrite_strings in general and for Gtk+ specific code.

* callproc.c (synch_process_death): Make const.
(Fcall_process): Make signame const.

* emacs.c (main): Pass char[] to putenv instead of literal.

* floatfns.c (matherr): Use a const char* variable for x->name.

* font.c (font_open_by_name): Make name const.

* font.h (font_open_by_name): Make name const.

* gtkutil.c (get_utf8_string): Always return an allocated string.
Parameter is const.
(create_dialog, xg_create_one_menuitem, create_menus)
(xg_item_label_same_p, xg_update_menu_item): Free result from
get_utf8_string.
(xg_separator_p, xg_item_label_same_p): label is const.

* gtkutil.h: Replace widget_value with struct _widget_value.
(enum button_type, struct _widget_value): Remove and use the one from
keyboard.h.

* keyboard.h (_widget_value): Add defined USE_GTK.  Replace Boolean
with unsigned char and XtPointer with void *.

* menu.c (Fx_popup_menu): error_name is const.

* menu.h (w32_menu_show, ns_menu_show, xmenu_show): error parameter
is const char **.

* w32menu.c (w32_menu_show):
* nsmenu.m (ns_menu_show): error parameter is const char **.

* process.h (synch_process_death): Is const char*.

* xmenu.c (Fx_popup_dialog): error_name is const char*.
(xmenu_show): error parameter is const char **. pane_string is const
char *.
(button_names): Is const char *.
(xdialog_show): error_name and pane_string is const.

* xrdb.c (get_system_app): Make path const and use char *p for non-const
char.

* xselect.c (Fx_get_atom_name): Use char empty[] instead of literal "".

* xsmfns.c (NOSPLASH_OPT): Change to char[].
(smc_save_yourself_CB): Do xstrdup on all ->type and ->name for
props.  Free them at the end.

* xterm.c (emacs_class): New char[] for EMACS_CLASS.
(xim_open_dpy, xim_initialize, xim_close_dpy): Use emacs_class.
(x_term_init): Use char[] display_opt and name_opt instead of
string literal. file is const char*.