Avoid the use of snprintf.

Avoid the use of snprintf.

* font.c (APPEND_SNPRINTF): Remove.
(font_unparse_xlfd):
* xterm.c (x_io_error_quitter):
Use esnprintf, not snprintf.  That way, we don't have to worry
about porting to ancient platforms that lack snprintf.
(x_term_init): Use sprintf, not snprintf.