override credential. New script is less crufty, I think.

Usage: genextendcred -t <days> [-e <days>] -s <slice> [-u <user> | -c <cert>]
       genextendcred -t <days> [-e <days>] -u <user> | -c <cert>
Options:
  -s    - Slice to use. If no user, issue to slice creator.
          In the absence of a slice, the target is the CM, allowing
          the user to extend any of his slivers.
  -u    - Issue to user
  -c    - Issue to user via his public certificate
  -t    - How many days to allow extension for
  -e    - How many days before credential expires; default 5 days

 Note that the renewsliver.py and renewslice.pl test scripts now take
 one of these credentials as an extra argument.