Use xstrdup and build_unibyte_string where applicable.

Use xstrdup and build_unibyte_string where applicable.
* alloc.c (xstrdup): Tiny cleanup.  Add eassert.
* xfns.c (x_window):
* xrdb.c (x_get_customization_string):
* xterm.c (xim_initialize):
* w32fns.c (w32_window): Use xstrdup.
(w32_display_monitor_attributes_list):
* emacs.c (init_cmdargs):
* keyboard.c (PUSH_C_STR):
* nsfont.m (nsfont_open):
* sysdep.c (system_process_attributes):
* w32.c (system_process_attributes):
* xdisp.c (message1, message1_nolog): Use build_unibyte_string.