1. Added a 'Z' option to specify the length of a statically allocated lease.
2. Added a 'default' clause to the options switch so as to catch invalid
   options.
3. Added options 'S' and 'r' to the usage() description.

In retrospect, a static lease time would have been unnecessary if the tests
of StaticLease against 'minlease' had been in the opposite direction.  There
are four of these and it's not altogether clear what their purpose is.  As
the code stands now, it seems to resolve everything except the unfortunate
choice of option letter, 'S" having been already taken.

Also, there is such a thing as MCLT (I don't want to look it up) in recent
RFC(s) and no implementation that I can see in this server.  It may be worth
looking into.

And...  Thanks for such easy source to modify :-)