Fix running from PATH and add better usage/manpage (#5)

* Misc changes

- Don't segfault when ran from PATH
- Remove useless `-u` arg from usage
- Elaborate on usage

* Add manpage

* usage: Use spaces not tabs

* Add snaputil -o

This will query the IORegistry to determine what the system apfs
snapshot's name should be.

* Update manpage

Co-authored-by: Cameron Katri <me@cameronkatri.com>

prototype, D script for debugging, initial snapUtil