Switch vmnetx-generate command line parsing to argparse
Improve documentation

- Update last-modified date on manpage
- List -nu flags for package subcommand summary
- Slightly rephrase vmnetx-generate create -h description
- Use domain-xml instead of domainxml for arguments
- Use original -n/-u help texts
Trim down command help texts.
Change argparse variable names, small documentation fix
Merge branch 'argparse' of https://github.com/jaharkes/vmnetx
vmnetx-generate: Fix --version
vmnetx-generate: Clean up subcommand descriptions
vmnetx-generate.1: Move subcommands into MODES section
vmnetx-generate.1: Fix SYNOPSIS formatting