veyron/security/agent: Switch to ssh terminal for password input.
veyron/security/agent: Switch to ssh terminal for password input.

* This prevents the password from being stored in memory in a string
  until the GC comes, since terminal.ReadPassword returns []byte.
* terminal.ReadPassword does not have signal handling so had to add that
  to turn echo back on in terminal if termination signal is received.

Change-Id: I560160a0c291de6f525f17a459cefe69c4d705c1

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref