Implement suspend/resume callbacks for the conversation function.

Implement suspend/resume callbacks for the conversation function.
If suspended, close the timestamp file (dropping all locks).  On
resume, lock the record before reading the password.

For this to work properly we need to be able to run th callback
when tsetattr() suspends us, not just when the user does.  To
accomplish this the term_* functions now return EINTR if SIGTTOU
would be generated.  The caller now has to restart the term_*
function (and send itself SIGTTOU) instead of it being done
automatically.