Added more support for account and password expiration

Added more support for account and password expiration

* Fixed bugs in the existing expiration command for Heimdal, and added
  it to the help command for both MIT and Heimdal.
* Added pwexpiration command that works like the expiration command, but
  for password expiration.
* Added check_expire command that will return expiration times in GMT
  for either account or password expiration.

The commands have been tested against Heimdal, though not yet against MIT.