Add support for actual hostnames in MPD_HOST

[Polochon-street]

No description provided.

[toofar]

I cherry picked and tested and it worked for me, thank you!

I tested:

• remote host by hostname and IP address with and without MPD_PORT set
• local host by hostname and IP address with and without MPD_PORT
• unix socket
• failed connections for TCP (connection refused) and unix socket (no such file or directory)

But this is where MPD parses it: https://github.com/MusicPlayerDaemon/MPD/blob/master/src/config/Net.cxx#L14-L16
So you might want to match that. Looks like paths can theoretically start with ~ and @, examples are in the docs here: https://github.com/MusicPlayerDaemon/MPD/blob/c71e586c530e1d066efde7c2ca40c363f74100c7/doc/user.rst#L779

I'm not sure if rust supports whatever those "abstract sockets" are. rust-lang/rust#14194 is marked as done but apparently you are supposed to pass the name as null? Or the first byte as null? I tried that and it doesn't seem to work (failed to lookup address information: Name or service not known).

[Polochon-street]

thanks a lot for double-checking - I could make it work with abstract sockets by following rust-lang/rust#42048 (comment)
I tested the various use cases, and everything seems to work 🤞 I'll merge it :)