Refactor commands to support different backends

Therefore also the `sysv` backend is intoduced to prepare for future
support. The man page and the readme are updated so far.

src/main.rs

@@ -4,7 +4,15 @@ use clap::{crate_authors, crate_version, AppSettings, Clap};
 mod posix;
 
 #[derive(Clap, Debug)]
-enum Command {
+enum Backend {
+    /// Handle POSIX message queues
+    Posix(PosixCommand),
+    /// Handle SysV message queues
+    Sysv(SysvCommand),
+}
+
+#[derive(Clap, Debug)]
+enum PosixCommand {
     Create(posix::Create),
     Info(posix::Info),
     List(posix::List),
@@ -13,6 +21,10 @@ enum Command {
     Recv(posix::Recv),
 }
 
+#[derive(Clap, Debug)]
+enum SysvCommand {
+}
+
 #[derive(Clap, Debug)]
 #[clap(
     version = crate_version!(),
@@ -25,8 +37,9 @@ struct Opts {
     /// Produce verbose output, multiple -v options increase the verbosity (max. 3)
     #[clap(short, long, global = true, parse(from_occurrences))]
     verbose: u32,
+    /// Backend to be used
     #[clap(subcommand)]
-    command: Command,
+    backend: Backend,
 }
 
 fn main() -> Result<()> {
@@ -42,13 +55,17 @@ fn main() -> Result<()> {
     ))
     .init();
 
-    match opts.command {
-        Command::Create(c) => c.run()?,
-        Command::Info(i) => i.run()?,
-        Command::Unlink(u) => u.run()?,
-        Command::Send(s) => s.run()?,
-        Command::Recv(r) => r.run()?,
-        Command::List(l) => l.run()?,
+    match opts.backend {
+        Backend::Posix(p) => match p {
+            PosixCommand::Create(c) => c.run()?,
+            PosixCommand::Info(i) => i.run()?,
+            PosixCommand::List(l) => l.run()?,
+            PosixCommand::Unlink(u) => u.run()?,
+            PosixCommand::Send(s) => s.run()?,
+            PosixCommand::Recv(r) => r.run()?,
+        },
+        Backend::Sysv(s) => match s {
+        },
     }
 
     Ok(())