Fix scm_to_utf8_stringn to return the length in bytes, et al

Fix scm_to_utf8_stringn to return the length in bytes, et al

* libguile/strings.c (u32_u8_length_in_bytes): Internal static function
  renamed from u32_u8_strlen, whose name was potentially confusing.  For
  added safety, handle everything that can be encoded in the more
  general UTF-8 encoding: up to six bytes for each code point, with code
  points up to 2^31-1.

  (scm_to_utf8_stringn): NUL-terminate only if (lenp == NULL).
  If (lenp != NULL) return the length in bytes in *lenp.