Refactor: Don't use po_gram_error_at_line outside the PO file reader.
* gettext-tools/src/msgcmp.c: Include po-xerror.h, xvasprintf.h.
(match_domain, compare): Use po_xerror, po_xerror2 instead of
po_gram_error_at_line.
* gettext-tools/src/msgfmt.c: Include po-xerror.h.
(msgfmt_set_domain): Use po_xerror instead of po_gram_error_at_line.
* gettext-tools/src/msgmerge.c: Include xvasprintf.h.
(match_domain): Use po_xerror, po_xerror2 instead of po_gram_error_at_line.
* gettext-tools/src/read-catalog.c (default_set_domain): Use po_xerror instead
of po_gram_error_at_line.
* gettext-tools/src/xgettext.c: Include po-xerror.h.
(exclude_directive_domain): Use po_xerror instead of po_gram_error_at_line.
