Add FreeBSD 'zpool labelclear' command

The FreeBSD implementation of zfs adds the 'zpool labelclear'
command.  Since this functionality is helpful and straight
forward to add it is being included in ZoL.

References:
  freebsd/freebsd@119a041dc9

Ported-by: Dmitry Khasanov <pik4ez@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1126

man/man8/zpool.8

@@ -92,6 +92,11 @@ zpool \- configures ZFS storage pools
 \fBzpool iostat\fR [\fB-T\fR u | d ] [\fB-v\fR] [\fIpool\fR] ... [\fIinterval\fR[\fIcount\fR]]
 .fi
 
+.LP
+.nf
+\fBzpool labelclear\fR [\fB-f\fR] \fIdevice\fR
+.fi
+
 .LP
 .nf
 \fBzpool list\fR [\fB-Hv\fR] [\fB-o\fR \fIproperty\fR[,...]] [\fIpool\fR] ...
@@ -1426,6 +1431,28 @@ Verbose statistics. Reports usage statistics for individual \fIvdevs\fR within t
 
 .RE
 
+.sp
+.ne 2
+.mk
+.na
+\fB\fBzpool labelclear\fR [\fB-f\fR] \fIdevice\fR
+.ad
+.sp .6
+.RS 4n
+Removes ZFS label information from the specified device. The device must not be part of an active pool configuration.
+.sp
+.ne 2
+.mk
+.na
+\fB\fB-f\fR\fR
+.ad
+.RS 12n
+.rt
+Treat exported or foreign devices as inactive.
+.RE
+
+.RE
+
 .sp
 .ne 2
 .mk