ssh-proxy: add scp and rsync support

ssh-generator and ssh-proxy are great features, it is very handy to be
able to do:

    ssh vsock/1

But, because of the '/' used as a separator, scp and rsync don't
interpret 'vsock/<CID>' as a hostname, e.g.

    $ scp /etc/machine-id vsock/2222:.
    cp: cannot create regular file 'vsock/2222:.': No such file or directory

    $ rsync /etc/machine-id vsock/2222:.
    rsync: [Receiver] change_dir#3 "(...)/vsock" failed: No such file or directory (2)
    rsync error: errors selecting input/output files, dirs (code 3) at main.c(829) [Receiver=3.4.1]

An alternative is to use ',' as separator, e.g.

    $ scp /etc/machine-id vsock,2222:.

This is what is being suggested here. The names with '/' are kept not to
break anything here.

Others are possible: '%', '=', '#', '@', ':', etc. As mentioned in
commit 0abd510f7f ("ssh-proxy: add ssh ProxyCommand tool that can
connect to AF_UNIX + AF_VSOCK sockets"), it is better to avoid ':' as it
is already taken by SSH itself when doing sftp, and "@" is already taken
for separating the user name. '#' will cause some issues with some
shells like ZSH when quotes are not used.

src/ssh-generator/ssh-proxy.c

@@ -175,6 +175,15 @@ static int process_machine(const char *machine, const char *port) {
         return process_vsock_cid(cid, port);
 }
 
+static char *startswith_sep(const char *s, const char *prefix) {
+        const char *p = startswith(s, prefix);
+
+        if (p && IN_SET(*p, '/', ','))
+                return (char*) p + 1;
+
+        return NULL;
+}
+
 static int run(int argc, char* argv[]) {
 
         log_setup();
@@ -184,19 +193,19 @@ static int run(int argc, char* argv[]) {
 
         const char *host = argv[1], *port = argv[2];
 
-        const char *p = startswith(host, "vsock/");
+        const char *p = startswith_sep(host, "vsock");
         if (p)
                 return process_vsock_string(p, port);
 
-        p = startswith(host, "unix/");
+        p = startswith_sep(host, "unix");
         if (p)
                 return process_unix(p);
 
-        p = startswith(host, "vsock-mux/");
+        p = startswith_sep(host, "vsock-mux");
         if (p)
                 return process_vsock_mux(p, port);
 
-        p = startswith(host, "machine/");
+        p = startswith_sep(host, "machine");
         if (p)
                 return process_machine(p, port);