Cache vattr in psshfs's setattr.

This means within the cache window, a setattr that wouldn't change the
remote file's attributes from our current view of them will not be
relayed to the server and wait for the server to answer.  Thus, e.g., a
process with a periodic timer interrupt that calls open(2) in a loop
can make progress with much higher probability than without caching.

XXX The test case doesn't work, so it's currently disabled.  It needs
to stop the child of sshd that is handling an sftp session, not sshd
itself, and it's not obvious how to do that.

ok pooka

(riastradh)