Update after gnulib changed.

Update after gnulib changed.

* gettext-runtime/src/envsubst.c (subst_from_stdin): Use sd_ function prefix
instead of string_desc_ function prefix.
* gettext-tools/src/its.c (set_doc_encoding_utf8, _its_is_valid_simple_gen_xml,
is_valid_xhtml_element, is_valid_html_element, is_no_end_html_element):
Likewise.
* gettext-tools/src/msgl-ascii.c (is_ascii_string_desc): Likewise.
* gettext-tools/src/msgl-iconv.c (convert_string_desc_directly): Likewise.
* gettext-tools/src/read-desktop.c (desktop_lex): Likewise.
* gettext-tools/src/x-awk.c (phase2_getc, phase3_get): Likewise.
* gettext-tools/src/x-c.c (phase5_get): Likewise.
* gettext-tools/src/x-perl.c (get_here_document, extract_quotelike_pass1,
extract_quotelike_pass1_utf8, extract_quotelike, extract_triple_quotelike,
extract_quotelike_pass3, extract_variable, interpolate_keywords, x_perl_prelex):
Likewise.
* gettext-tools/src/x-php.c (phase4_get): Likewise.
* gettext-tools/src/x-rst.c (parse_integer): Likewise.
* gettext-tools/src/x-ycp.c (phase2_getc, phase5_get): Likewise.
* gettext-tools/src/xg-encoding.c (string_desc_from_current_source_encoding):
Likewise.