Add rest of socket options for unix

[Kleinmarb]

There is #132 and #183 but these socket options are still missing:
SO_DONTROUTE,
SO_DEBUG,
SO_RCVLOWAT,
SO_SNDLOWAT,

I'm going to add these in unix.rs.

[Thomasdezeeuw]

There are many options missing, having a generic issue to "add the rest" I don't think will be helpful. Prs to add new options are welcome though.

[Kleinmarb]

Ok I thought these were all missing SO_* on Linux but apparently the website that listed them was incomplete as well.

[Thomasdezeeuw]

Closing this as it's too broad.