string-desc: Distinguish writable strings from read-only strings. * lib/string-desc.h (HAVE_STATEMENT_EXPRESSIONS): New macro. (rw_string_desc_t): New type. (string_desc_t) [HAVE_STATEMENT_EXPRESSIONS]: Change field _data from 'char *' to 'const char *'. (sd_readonly, sd_readwrite): New inline functions. (sd_length): Define through a macro with _Generic. (sd_char_at): Define through a macro and an inline function. (sd_data, sd_is_empty): Define through a macro with _Generic. (sd_equals, sd_startswith, sd_endswith, sd_cmp, sd_c_casecmp, sd_index, sd_last_index, sd_contains): Define through a macro. (sd_new_addr): Define through a macro with _Generic. (sd_substring, sd_write, sd_fwrite): Define through a macro. (sd_new, sd_new_filled): Change parameter type. (sd_copy): Define through a macro. (sd_concat): Change parameter type. (sd_c): Define through a macro. (sd_set_char_at, sd_fill): Change parameter type. (sd_overwrite): Define through a macro. (sd_free): Change parameter type. * lib/string-desc.c (_sd_equals): Renamed from sd_equals. Take scalar parameters. (_sd_startswith): Renamed from sd_startswith. Take scalar parameters. (_sd_endswith): Renamed from sd_endswith. Take scalar parameters. (_sd_cmp): Renamed from sd_cmp. Take scalar parameters. (_sd_c_casecmp): Renamed from sd_c_casecmp. Take scalar parameters. (_sd_index): Renamed from sd_index. Take scalar parameters. (_sd_last_index): Renamed from sd_last_index. Take scalar parameters. (_sd_new_addr, _rwsd_new_addr): Renamed from sd_new_addr. (sd_substring): Remove function. (_sd_write): Renamed from sd_write. Take scalar parameters. (_sd_fwrite): Renamed from sd_fwrite. Take scalar parameters. (sd_new, sd_new_filled): Change parameter type. (_sd_copy): Renamed from sd_copy. Change parameter type. Take scalar parameters. (sd_concat): Change parameter type. (_sd_c): Renamed from sd_c. Take scalar parameters. (sd_set_char_at, sd_fill): Change parameter type. (_sd_overwrite): Renamed from sd_overwrite. Change parameter type. Take scalar parameters. (sd_free): Change parameter type. * lib/string-desc-contains.c (_sd_contains): Renamed from sd_contains. Take scalar parameters. * lib/xstring-desc.h (xsd_new, xsd_new_filled, xsd_copy, xsd_concat): Change return type to rw_string_desc_t. (xsd_c): Define through a macro. * lib/xstring-desc.c (xsd_concat): Change return type to rw_string_desc_t. * doc/string-desc.texi (Handling strings with NUL characters): Mention rw_string_desc_t and the sd_readonly() function. * lib/string-buffer.h (sb_dupfree, sb_xdupfree): Change return type to rw_string_desc_t. * lib/string-buffer.c (sb_contents): Add a cast to 'const char *'. (sb_dupfree): Change return type to rw_string_desc_t. * lib/xstring-buffer.c (sb_xdupfree): Change return type to rw_string_desc_t. * lib/string-buffer-reversed.h (sbr_dupfree, sbr_xdupfree): Change return type to rw_string_desc_t. * lib/string-buffer-reversed.c (sbr_contents): Add a cast to 'const char *'. (sbr_dupfree): Change return type to rw_string_desc_t. * lib/xstring-buffer-reversed.c (sbr_xdupfree): Change return type to rw_string_desc_t. * tests/test-string-desc.c (main): Use type rw_string_desc_t as appropriate. * tests/test-xstring-desc.c (main): Likewise. * tests/test-sf-istream.c (main): Remove cast in sd_new_addr argument. * tests/test-sfl-istream.c (main): Likewise. * NEWS: Mention the change.