This is the path in which pinned maps are created or resolved. It
isn't actually the path for any specific map itself. This rename
makes way for a method `set_map_pin_path` that actually specifies
the pin path for a specific map.
Rather than support N async runtimes, push this to the user. The
relevant types (`PerfEventArrayBuffer` and `RingBuffer`) implement
`As{,Raw}Fd` which is sufficient with integration with tokio, smol, and
other async runtimes.