Implement and use new `simple-proc-net` crate for `/proc/net` reading

• #61 (closed) shows that there are issues with the procfs crate we're using on some platforms (it's validating the 'state' parameter and finding an unexpected value).
• I don't really want to maintain a fork of that crate to make it more permissive, and we really don't need much functionality here, so just reimplement the bare minimum in a new simple-proc-net crate.
  • This gets you (local address, remote address, state, uid), which is fine for our purposes.
  • The crate also has zero dependencies, and unit tests!
• Also, implement /proc/net isolation in the onionmasq Linux test crate, too.