xgettext: Support up to 4 format string parsers at the same time.

xgettext: Support up to 4 format string parsers at the same time.

* gettext-tools/src/xg-arglist-context.h (struct flag_context_ty): Add fields
is_format4, pass_format4.
* gettext-tools/src/xg-arglist-context.c (null_context, passthrough_context,
inherited_context, passthrough_context_circular_list,
flag_context_list_table_add): Update.
* gettext-tools/src/xg-message.c (set_format_flags_from_context,
decide_is_format, remember_a_message_plural): Update.
* gettext-tools/src/xgettext.h (current_formatstring_parser4): New declaration.
* gettext-tools/src/xgettext.c (struct extractor_ty): Add field
formatstring_parser4.
(current_formatstring_parser4): New variable.
(main, extract_from_file, language_to_extractor): Update.