Convert installperl to use Getopt::Long.

Convert installperl to use Getopt::Long.

This requires some special handling for the +v option. Fortunately, we can
cheat, because we know that no other non-options are acceptable. This does
change the behaviour slightly from before:

* single letter options now also have long names
* invalid options are now rejected
* command line arguments that are not options are now rejected

However, the behaviour for all valid documented invocations is unchanged.

Also, add a --help option to display the usage message.