Refactor: Remove code duplication.

Refactor: Remove code duplication.

* gettext-tools/src/po-xerror.h: Include xerror-handler.h. Remove all
declarations.
(PO_SEVERITY_*): Define as aliases of CAT_SEVERITY_*.
(po_xerror, po_xerror2): New macros.
* gettext-tools/src/po-xerror.c: Remove file.
* gettext-tools/src/Makefile.am (COMMON_SOURCE): Remove po-xerror.c.
* gettext-tools/src/msgmerge.c (silent_xerror2): New function.
(match_domain): Pass a stack-allocated xerror_handler to check_plural_eval.
* gettext-tools/woe32dll/gettextsrc-exports.c (po_xerror, po_xerror2): Remove
exports.
* gettext-tools/po/POTFILES.in: Add src/xerror-handler.c. Remove
src/po-xerror.c.