bectl(8): Allow running a custom command in the 'jail' subcommand

Instead of always running /bin/sh, allow the user to specify the command
to run. The jail is not removed when the command finishes. Meaning,
`bectl unjail` will still need to be run.

For example:

```
bectl jail newBE pkg upgrade
bectl ujail newBE
```

Submitted by:	Shawn Webb
Obtained from:	HardenedBSD (8b451014ab)

3 files changed, 17 insertions, 8 deletions

diff --git a/sbin/bectl/bectl.8 b/sbin/bectl/bectl.8
index 7c90eece3f3a..5cd4e0ed01be 100644
--- a/sbin/bectl/bectl.8
+++ b/sbin/bectl/bectl.8
@@ -18,7 +18,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 16, 2018
+.Dd August 17, 2018
 .Dt BECTL 8
 .Os
 .Sh NAME
@@ -54,6 +54,7 @@ jail
 .Oo Fl o Ar key Ns = Ns Ar value | Fl u Ar key Oc Ns ...
 .Ao Ar jailID | jailName Ac
 .Ao Ar bootenv Ac
+.Op Ar utility Op Ar argument ...
 .Nm
 list
 .Op Fl a
@@ -150,6 +151,7 @@ from
 .Oo Fl o Ar key Ns = Ns Ar value | Fl u Ar key Oc Ns ...
 .Ao Ar jailID | jailName Ac
 .Ao Ar bootenv Ac
+.Op Ar utility Op Ar argument ...
 .Pp
 Creates a jail of the given boot environment.
 Multiple
@@ -161,8 +163,16 @@ arguments may be specified.
 will set a jail parameter, and
 .Fl u
 will unset a jail parameter.
-By default, jails are created in interactive mode, with a shell being
+.Pp
+By default, jails are created in interactive mode and
+.Pa /bin/sh
+is
 executed within the jail.
+If
+.Ar utility
+is specified, it will be executed instead of
+.Pa /bin/sh .
+.Pp
 The
 .Fl b
 argument enables batch mode, thereby disabling interactive mode.
diff --git a/sbin/bectl/bectl.c b/sbin/bectl/bectl.c
index 0c685b9c3b01..9a0f398aa30c 100644
--- a/sbin/bectl/bectl.c
+++ b/sbin/bectl/bectl.c
@@ -77,7 +77,7 @@ usage(bool explicit)
 #if SOON
 	    "\tbectl add (path)*\n"
 #endif
-	    "\tbectl jail [-b] [ -o key=value | -u key ]... bootenv\n"
+	    "\tbectl jail [-b] [ -o key=value | -u key ]... bootenv [utility [argument ...]]\n"
 	    "\tbectl list [-a] [-D] [-H] [-s]\n"
 	    "\tbectl mount beName [mountpoint]\n"
 	    "\tbectl rename origBeName newBeName\n"
diff --git a/sbin/bectl/bectl_jail.c b/sbin/bectl/bectl_jail.c
index 335099da09b0..f0ea74c3177e 100644
--- a/sbin/bectl/bectl_jail.c
+++ b/sbin/bectl/bectl_jail.c
@@ -238,10 +238,6 @@ bectl_cmd_jail(int argc, char *argv[])
 		fprintf(stderr, "bectl jail: missing boot environment name\n");
 		return (usage(false));
 	}
-	if (argc > 2) {
-		fprintf(stderr, "bectl jail: too many arguments\n");
-		return (usage(false));
-	}
 
 	bootenv = argv[0];
 
@@ -284,7 +280,10 @@ bectl_cmd_jail(int argc, char *argv[])
 	if (interactive) {
 		/* We're attached within the jail... good bye! */
 		chdir("/");
-		execl("/bin/sh", "/bin/sh", NULL);
+		if (argc > 1)
+			execve(argv[1], &argv[1], NULL);
+		else
+			execl("/bin/sh", "/bin/sh", NULL);
 		return (1);
 	}