Update after gnulib changed.

Update after gnulib changed.

* gettext-tools/src/msgl-iconv.h (convert_string_desc_directly): Change result
type to rw_string_desc_t.
* gettext-tools/src/msgl-iconv.c (convert_string_desc_directly): Likewise.
* gettext-tools/src/its.c (_its_is_valid_simple_gen_xml): Omit casts to
'char *'. Don't use sd_c_casecmp, sd_cmp as function pointers.
* gettext-tools/src/xg-encoding.h (string_desc_from_current_source_encoding):
Change result type to rw_string_desc_t.
* gettext-tools/src/xg-encoding.c (from_current_source_encoding): Return
converted result directly, without a cast.
(string_desc_from_current_source_encoding): Change result type to
rw_string_desc_t. Return converted result directly, without a cast. Invoke
sd_readwrite.
* gettext-tools/src/x-perl.c (get_here_document): Add benign cast.
(extract_quotelike_pass1, extract_quotelike_pass1_utf8): Change result type to
rw_string_desc_t.
(extract_quotelike, extract_triple_quotelike): Update.
(extract_quotelike_pass3): Add benign casts.
(interpolate_keywords): Don't call sd_set_char_at on a string_desc_t.
* gettext-tools/src/x-rust.c (handle_comments, string_literal_value): Omit casts
to 'char *'. Add benign casts.
(extract_from_function_call, extract_from_function_call_like,
extract_from_node): Omit casts to 'char *'.