xgettext: Refactor.
* gettext-tools/src/xg-formatstring.h: New file, extracted from xgettext.h.
* gettext-tools/src/xg-formatstring.c: New file, extracted from xgettext.c.
* gettext-tools/src/FILES: Mention them.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add xg-formatstring.h.
(xgettext_SOURCES): Add xg-formatstring.c.
* gettext-tools/src/xgettext.h (current_formatstring_parser1,
current_formatstring_parser2, current_formatstring_parser3,
current_formatstring_parser4): Remove declarations.
* gettext-tools/src/xgettext.c: Include xg-formatstring.h.
(current_formatstring_parser1, current_formatstring_parser2,
current_formatstring_parser3, current_formatstring_parser4): Remove variables.
* gettext-tools/src/xg-message.c: Include xg-formatstring.h.
