Implement deletion of SysV IPC message queues

As usual also the readme and the man page are updated.

src/main.rs

@@ -25,6 +25,7 @@ enum PosixCommand {
 #[derive(Clap, Debug)]
 enum SysvCommand {
     Create(sysv::Create),
+    Unlink(sysv::Unlink),
 }
 
 #[derive(Clap, Debug)]
@@ -68,6 +69,7 @@ fn main() -> Result<()> {
         },
         Backend::Sysv(s) => match s {
             SysvCommand::Create(c) => c.run()?,
+            SysvCommand::Unlink(u) => u.run()?,
         },
     }