port locking refactor

port locking refactor

* libguile/ports.h (struct scm_t_port): Make the lock into a pointer
  field instead of an inline field.  It should be possible to make
  unlocked ports by having a NULL lock field.
  (scm_c_lock_port, scm_c_try_lock_port): Return the mutex if the port
  was actually locked.
  (scm_c_unlock_port): Remove.

* libguile/ports.c (scm_c_make_port_with_encoding): For now, leave
  `lock' set to 0.
  Change scm_c_lock_port callers to pay attention to the new API.

* libguile/print.c (scm_write, scm_display): Fix call to
  dynwind-lock-port for ports-with-print-states.