locking on port buffering operations

locking on port buffering operations

* libguile/ports.c (scm_fill_input_unlocked, scm_fill_input)
  (scm_end_input, scm_end_input_unlocked, scm_flush)
  (scm_flush_unlocked): Add locking and _unlocked variants.

* libguile/filesys.c:
* libguile/fports.c:
* libguile/gdbint.c:
* libguile/r6rs-ports.c:
* libguile/read.c:
* libguile/rw.c: Adapt callers to use _unlocked variants.