Fixes <https://bugs.gnu.org/35920>.
Reported by Christopher Lam <christopher.lck@gmail.com>.
* libguile/stime.c (scm_strftime): Use 'scm_to_locale_stringn' instead
of 'scm_to_utf8_stringn'.
(scm_strptime): Likewise, and use 'scm_string_length' instead of
'u8_strnlen'.
* test-suite/tests/time.test ("strftime")["strftime passes wide
characters"]: Wrap body in 'with-locale'.
["strftime fr_FR.utf8", "strftime fr_FR.iso88591"]: New tests.
("strptime")["strftime fr_FR.utf8", "strftime fr_FR.iso88591"]: New
tests.
* libguile/socket.c (scm_recv): receive the message without holding the
stringbuf writing lock
(scm_send): try to narrow a string before using it
* libguile/stime.c (strftime): convert string to UTF-8 so that it can
be safely passed to strftime
(strptime): convert input string to UTF-8 so that it can be safely
passed through strptime
* libguile/strings.c (narrow_stringbuf): new function
(scm_i_try_narrow_string): new function
* libguile/strings.h: new declaration for scm_i_try_narrow_string