bectl(3)/libbe(3): Allow BE root to be specified

Add an undocumented -r option preceding the bectl subcommand to specify a BE
root to operate out of. This will remain undocumented for now, as some
caveats apply:

- BEs cannot be activated in the pool that doesn't contain the rootfs
- bectl create cannot work out of the box without the -e option right now,
  since it defaults to the rootfs and cross-pool cloning doesn't work like
  that (IIRC)

Plumb the BE root through to libbe(3) so that some things -can- be done to
it, e.g.

bectl -r tank/ROOT create -e default upgrade
bectl -r tank/ROOT mount upgrade /mnt

this aides in some upgrade setups where rootfs is not necessarily ZFS, and
also makes it easier/possible to regression-test bectl when combined with a
file-backed zpool.

MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D18029

1 files changed, 8 insertions, 2 deletions

diff --git a/lib/libbe/be_info.c b/lib/libbe/be_info.c
index 8a6038009b82..e8869e6f94c8 100644
--- a/lib/libbe/be_info.c
+++ b/lib/libbe/be_info.c
@@ -42,7 +42,10 @@ const char *
 be_active_name(libbe_handle_t *lbh)
 {
 
-	return (strrchr(lbh->rootfs, '/') + sizeof(char));
+	if (*lbh->rootfs != '\0')
+		return (strrchr(lbh->rootfs, '/') + sizeof(char));
+	else
+		return (lbh->rootfs);
 }
 
 
@@ -63,7 +66,10 @@ const char *
 be_nextboot_name(libbe_handle_t *lbh)
 {
 
-	return (strrchr(lbh->bootfs, '/') + sizeof(char));
+	if (*lbh->bootfs != '\0')
+		return (strrchr(lbh->bootfs, '/') + sizeof(char));
+	else
+		return (lbh->bootfs);
 }