(io_listen): Use io_listen_fd.
(lsh_popd): Renamed safe_popd, and made non-static.
(lsh_pushd_fd): New function.
(lsh_pushd): Renamed safe_pushd, and use lsh_pushd_fd. Also added
arguments result and secret. Updated all callers.

Rev: src/io.c:1.169
Rev: src/io.h:1.91