add a ResourceManager to replace __del__ methods, and use it in SSHClient
to automatically close any open transport when the SSHClient is collected.
this won't work on Transport itself (to close the attached packetizer)
because Transport starts up its own thread, and the threading library
keeps a Transport object alive to run that thread. i think that's okay;
the SSHClient interface is meant to be the easier one, so that's the one
where it's important that some auto-cleanup is attempted.
(bug 69330)
in SFTPClient._auth, check for the existence of the rsa/dsa keys before
trying to open them, so that an I/O exception doesn't mask an earlier
one.
Robey Pointer