more flexible logging
some tweaks to make channels etc follow the logger setting of their parent
transport, so that setting the log channel for a paramiko transport will
cause all sub-logging to branch out from that channel.
also, close all open file handles when the sftp server ends.
cleanup & docs in sftp
add some more docs to SFTPHandle, and give a default implementation for
close() that's usually right. add a flush() to the default implementation
of write(). document that symlink's args in the sftp protocol are out of
order (the spec is wrong).
sftp server support!
finally check in sftp_handle (file handle abstraction), sftp_si (server
interface), and sftp_server (server implementation) -- all of which make
a roughly 90% implementation of server-side sftp.