Plug various leaks and fix input method initialization

Plug various leaks and fix input method initialization

* src/image.c (free_bitmap_record): Free bm->name correctly even
if the display connection has been closed.
* src/xfns.c (x_window): Use dpyinfo-specific `use_xim' flag.
* src/xterm.c (use_xim): Remove global variable.
(xim_destroy_callback): Free `xim_styles' if present, and set it
to NULL to be on the safe side.
(xim_open_dpy): Consult dpyinfo->use_xim instead.  Don't leak
`xim_styles' if an IM was previously opened.
(xim_initialize): Use dpyinfo-specific `use_xim' flag.
(xim_close_dpy): Check if `dpyinfo->xim_callback_data' is set
before unregistering the instantiation callback.
(x_term_init): Determine whether or not to use XIM for each
display opened, instead of using the resources of the last
display opened to toggle a global flag.
(x_delete_terminal): Always call `image_destroy_all_bitmaps' and
`xim_close_dpy'.  Free storage used to hold bitmap records.
* src/xterm.h (struct x_display_info): New field `use_xim'.