(Qunknown): New variable.

(Qunknown): New variable.
(init_charset_once): Intern and staticpro Qunknown.  Initialize
all elements of Vcharset_symbol_table to Qunknown.
(find_charset_in_str): New arg MULTIBYTE.  If it is zero, check
unibyte characters only.  For an invalid composition sequence, set
CHARSETS[1] to 1.
(Ffind_charset_region): Call find_charset_in_str with an
appropriate MULTIBYTE arg.  If undefined charsets are found,
include `unknown' is the return value.
(Ffind_charset_string): Likewise.
(Fsplit_char): If CHAR is invalid, return `(unknown CHAR)'.
(str_cmpchar_id): Max composite character code should be less than
GENERIC_COMPOSITION_CHAR.