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.