chpwd, userv.rc: Change Userv service protocol to parse options properly.

Options passed apparently to the service were actually being handled at
top level, so you got the wrong help text if you said `userv chpwd list
-h', for example.  This is very bad.

To fix this, we require the userv configuration to provide an explicit
non-option token to terminate top-level option handling at the right
time.