Support SystemD LISTEN_FDS.

Receive pre-bound UDP and TCP sockets via the LISTEN_FDS environment
variable.

Author: ximon18

Cargo.toml

@@ -21,6 +21,6 @@ toml_edit       = "0.22"
 
 
 [target.'cfg(unix)'.dependencies]
-nix             = { version = "0.29", features = [ "fs", "process", "user" ] }
+nix             = { version = "0.29", features = [ "fs", "net", "process", "socket", "user" ] }
 syslog          = "7"