Use const char* instead of char*.

Use const char* instead of char*.
Reduce the number of warnings with -Wwrite-strings.
* src/xrdb.c (get_environ_db, get_system_name):
* src/unexelf.c (find_section):
* src/term.c (string_cost, string_cost_one_line, per_line_cost)
(get_named_tty, init_tty):
* src/sysdep.c (sys_subshell):
* src/sound.c (sound_perror, sound_warning, vox_open, vox_init)
(alsa_sound_perror, alsa_open, alsa_configure, alsa_init):
* src/search.c (Freplace_match):
* src/process.c (Fmake_network_process, send_process, init_process):
* src/lread.c (Fload, init_lread):
* src/keymap.c (Fdescribe_buffer_bindings, describe_map_tree):
* src/keyboard.c (parse_tool_bar_item, struct event_head):
* src/gtkutil.h (xg_get_font_name):
* src/gtkutil.c (get_dialog_title, create_dialog, xg_get_font_name)
(make_widget_for_menu_item, make_menu_item, create_menus)
(xg_make_tool_item):
* src/font.c (parse_matrix, font_parse_name):
* src/floatfns.c (rounding_driver, float_error_fn_name):
* src/filelock.c (get_boot_time_1, lock_file_1):
* src/fileio.c (barf_or_query_if_file_exists, check_writable):
* src/editfns.c (get_system_name, get_operating_system_release)
(Fencode_time, Fset_time_zone_rule):
* src/dispextern.h (string_cost, per_line_cost, get_named_tty, init_tty):
* src/buffer.c (defvar_per_buffer): Use const.