OpenZFS 7280 - Allow changing global libzpool variables in zdb and ztest through command line

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7280
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0e60744
Closes #5676

man/man8/zdb.8

@@ -21,7 +21,7 @@
 .SH "SYNOPSIS"
 \fBzdb\fR [-CumdibcsDvhLMXFPAG] [-e [-p \fIpath\fR...]] [-t \fItxg\fR]
     [-U \fIcache\fR] [-I \fIinflight I/Os\fR] [-x \fIdumpdir\fR]
-    [\fIpoolname\fR [\fIobject\fR ...]]
+    [-o \fIvar\fR=\fIvalue\fR] ... [\fIpoolname\fR [\fIobject\fR ...]]
 
 .P
 \fBzdb\fR [-divPA] [-e [-p \fIpath\fR...]] [-U \fIcache\fR]
@@ -420,6 +420,18 @@ default value is 200. This option affects the performance of the \fB-c\fR
 option.
 .RE
 
+.sp
+.ne 2
+.na
+\fB-o \fIvar\fR=\fIvalue\fR ... \fR
+.ad
+.sp .6
+.RS 4n
+Set the given global libzpool variable to the provided value. The value must
+be an unsigned 32-bit integer. Currently only little-endian systems are
+supported to avoid accidentally setting the high 32 bits of 64-bit variables.
+.RE
+
 .sp
 .ne 2
 .na