(encode_designation_at_bol): Fix bug of finding graphic

(encode_designation_at_bol): Fix bug of finding graphic
registers which should be designated at bol.
(Qsafe_charset): New variable.
(syms_of_coding): Initialize and staticpro it.
(detect_coding_iso2022): Handle SS2 and SS3 correctly.
(DECODE_ISO_CHARACTER): Recover from incorrect encoding in less
dangerous way.
(ENCODE_DESIGNATION): Get charset revision number by
CODING_SPEC_ISO_REVISION_NUMBER.
(setup_coding_system): Initialize the member safe_charsets from
the coding systems's safe-charsets property.  Initialize the
member charset_revision_number of struct iso2022_spec.
(ENCODE_ISO_CHARACTER_DIMENSION1): Adjusted for the change of
`safe_charsets' member.
(ENCODE_ISO_CHARACTER_DIMENSION2): Likewise.
(code_convert_region): Restore the current point after calling a
function in coding->post_read_conversion.