Include mb-1byte.h in unibyte-XEmacs.
(MAX_EMCHAR_LEN): Moved to mb-1byte.h.
(VALID_CHARPTR_P): Moved to mb-*byte.h.
(VALIDATE_CHARPTR_BACKWARD): Likewise.
(VALIDATE_CHARPTR_FORWARD): Likewise.
(simple_charptr_emchar): Moved to mb-multibyte.h.
(simple_set_charptr_emchar): Likewise.
(simple_charptr_copy_char): Likewise.
(non_ascii_charptr_emchar): Likewise.
(non_ascii_set_charptr_emchar): Likewise.
(non_ascii_charptr_copy_char): Likewise.
(charptr_emchar): Moved to mb-*byte.h.
(set_charptr_emchar): Likewise.
(charptr_copy_char): Likewise.