Add xerror-handler.
* gettext-tools/src/xerror-handler.h: New file, based on gettext-tools/src/po-xerror.h.
* gettext-tools/src/xerror-handler.c: New file, based on gettext-tools/src/po-xerror.c.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add xerror-handler.h.
(COMMON_SOURCE): Add xerror-handler.c.
* gettext-tools/src/FILES: Update.
* gettext-tools/woe32dll/gettextsrc-exports.c (textmode_xerror_handler_struct):
New export.
