Make watcher's path absolute for notify's MacOS fsevent implementation

[voidbuf]

This is a proposed fix for #154
I have chosen to replicate the logic present in notify's inotify implementation. In the notify crate, these checks make it so that there is no error when presenting a watcher with an empty path ("") on Linux, but they are missing from MacOS's fsevent implementation, and therefore I propose to always offer the watcher an absolute path.

Another possible solution is to handle the error returned from this call:

watcher.watch(parent_path, RecursiveMode::NonRecursive)?;

using something like

if let Err(e) = watcher.watch(parent_path, RecursiveMode::NonRecursive) {
  // handle...
}

but this means that one could not run rathole on MacOS using this syntax: rathole config.toml

[rapiz1]

Nice catch. Thanks for your contribution!