xgettext: Provide a way to interpret string literals lazily
* xgettext.c (extract_ty): New field 'literalstring_parser'.
(current_literalstring_parser): New variable.
(extract_from_file): Set 'current_literalstring_parser'.
(savable_comment_convert_encoding): New function.
(arglist_parser_remember_literal): New function.
(arglist_parser_remember): Call 'arglist_parser_remember_literal'.
(arglist_parser_done): Call literalstring_parser on msgctxt,
msgid, and msgid_plural before calling 'remember_a_message';
convert encoding of msgid_comment.
* x-*.h: Register 'literalstring_parser' through SCANNER_*.
* xgettext.h (enum literalstring_escape_type): New enum.
(struct literalstring_parser): New struct.
(struct partial_call): New fields 'msgctxt_escape',
'msgid_escape', and 'msgid_plural_escape'.
(arglist_parser_remember_literal): New function declaration.
(savable_comment_convert_encoding): New function declaration.
