exec: expose --preserve-fds

The implementation is already there, we only need to add the CLI
option and pass it down.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

exec.go

@@ -89,6 +89,10 @@ following will output a list of processes running in the container:
 			Usage:  "disable the use of the subreaper used to reap reparented processes",
 			Hidden: true,
 		},
+		cli.IntFlag{
+			Name:  "preserve-fds",
+			Usage: "Pass N additional file descriptors to the container (stdio + $LISTEN_FDS + N in total)",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if err := checkArgs(context, 1, minArgs); err != nil {
@@ -141,6 +145,7 @@ func execProcess(context *cli.Context) (int, error) {
 		pidFile:         context.String("pid-file"),
 		action:          CT_ACT_RUN,
 		init:            false,
+		preserveFDs:     context.Int("preserve-fds"),
 	}
 	return r.run(p)
 }