native: Unlink unix socket paths on drop

This prevents unix sockets from remaining on the system all over the place, and
more closely mirrors the behavior of libuv and windows pipes.
auto merge of #13690 : alexcrichton/rust/unlink-unix-pipe, r=brson

This prevents unix sockets from remaining on the system all over the place, and
more closely mirrors the behavior of libuv and windows pipes.