multiple operations. For example, you can now remove multiple VLANs with
one command, like:
snmpit -o foo -o bar

You can now give more than one -i option, so that you can give a list of
more than one switch to operate on, like:
snmpit -i cisco3 -i cisco4 -l

Converted from using Getopt::Std to Getopt::Long, which is more flexible and
better documented.

All the 'worker' ( do*() ) functions now take a list of stacks as the first
argument, rather than using a global @stacks variable.

Fixed up the usage message, which was out of date in some cases, and
innacurate in others.