Avoid crashes when X fonts are erroneously freed on reused X

Avoid crashes when X fonts are erroneously freed on reused X
'Display *' connection data (Bug#16069).  Note that X font
resources still may be leaked, but currently there is no way
to completely avoid it.
* xterm.h (struct x_display_info): New member x_id.  Add comments.
* xterm.c (x_display_id): New variable.
(x_term_init): Assign identifier to each opened X connection.
* xfont.c (struct xfont): New member x_display_id.
(xfont_open): Initialize it with frame's display id.
(xfont_close): Check whether font's display id matches the one
recorded for the given display.  Adjust comment.
* xftfont.c (struct xftfont_info):
(xftfont_open, xftfont_close): Exactly as above with xfont stuff.