xgettext: Add new check bullet-unicode

xgettext: Add new check bullet-unicode

* gettext-tools/src/xgettext.c (main): Generalize --check option
handling.
* gettext-tools/src/message.h (enum syntax_check_type): New enum value
sc_bullet_unicode.
(NSYNTAXCHECKS): Increment.
* gettext-tools/src/message.c (syntax_check_name): Add name for
sc_bullet_unicode.
* gettext-tools/src/msgl-check.c (struct bullet_ty, struct
bullet_stack_ty): New struct.
(bullet_stack): New variable.
(syntax_check_bullet_unicode): New function.
(sc_funcs): Register syntax_check_bullet_unicode as a check function for
sc_bullet_unicode.
* gettext-tools/tests/xgettext-14: Add tests for --check=bullet-unicode