Modify socket and time functions for wide strings

Modify socket and time functions for wide strings

* 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