two additional updates for EXPIRE_PASSWORDS=0 mode
In User.pm Create(), only apply the default expire time of 1 year to pswd_expires if EXPIRE_PASSWORDS is true.
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.