Add getFd method to FdInputStream and FdOutputStream
Add getFd method to FdInputStream and FdOutputStream

These methods are useful if you're receiving a generic stream in
application code and would like to check some non-stream behavior about
the file (e.g. stat or isatty).  Of course, downcasting can fail and
code should not rely on the file descriptor being available.

mologie/capnproto,mologie/capnproto,mologie/capnproto