its: Support DocBook XML, part 2.
Reported by Guillem Jover <guillem@hadrons.org>
at <https://savannah.gnu.org/bugs/?52415>.
* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add xstring-desc.
* gettext-tools/src/its.c: Include xstring-desc.h, c-ctype.h, unistr.h.
(SIZEOF): New macro.
(its_extension_escape_rule_constructor, its_extension_escape_rule_eval): Handle
the 'unescape-if' attribute.
(starts_with_character_reference): Add an ucs_p parameter.
(_its_encode_special_chars_for_merge): Update.
(set_doc_encoding_utf8, _its_is_valid_simple_gen_xml, _its_is_valid_simple_xml,
is_valid_xhtml_element, _its_is_valid_simple_xhtml, is_valid_html_element,
is_no_end_html_element, _its_is_valid_simple_html, _its_set_simple_xml_content,
_its_set_simple_html_content): New functions.
(its_merge_context_merge_node): Implement the 'unescape-if' attribute.
* gettext-tools/src/its-extensions.xsd (EscapeRuleType): Add 'unescape-if'
attribute.
* gettext-tools/its/docbook4.its: Add an <escapeRule>.
* gettext-tools/its/docbook5.its: Likewise.
* gettext-tools/tests/xgettext-docbook-1: New file.
* gettext-tools/tests/msgfmt-xml-5: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
* gettext-tools/doc/gettext.texi (ITS Rules): Document the 'unescape-if'
attribute of the escapeRule.
* NEWS: Mention the change.
