1. In User.pm Create(), only apply the default expire time of 1 year
to pswd_expires if EXPIRE_PASSWORDS is true.

2. In tbacct's passwd command: the current behavior is that we set
the pswd_expires time to "now" if we are changing the the password
of someone else's account.   this patch adds a new "-e" flag that,
if specified, uses the default expiration policy instead of now.
The rational for this change is to allow scripts to import encrypted
passwords from external account management systems and apply them
to emulab using "tbacct passwd" without forcing an immediate change.