zdb: Introduce -V for verbatim import

When given a pool name via -e, zdb would attempt an import. If it
failed, then it would attempt a verbatim import. This behavior is
not always desirable so a -V switch is added to zdb to control the
behavior. When specified, a verbatim import is done. Otherwise,
the behavior is as it was previously, except no verbatim import
is done on failure.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2372

man/man8/zdb.8

@@ -19,7 +19,7 @@
 \fBzdb\fR - Display zpool debugging and consistency information
 
 .SH "SYNOPSIS"
-\fBzdb\fR [-CumdibcsDvhLXFPA] [-e [-p \fIpath\fR...]] [-t \fItxg\fR]
+\fBzdb\fR [-CumdibcsDvhLVXFPA] [-e [-p \fIpath\fR...]] [-t \fItxg\fR]
     [-U \fIcache\fR] [-M \fIinflight I/Os\fR] [\fIpoolname\fR
     [\fIobject\fR ...]]
 
@@ -429,6 +429,17 @@ Attempt \'extreme\' transaction rewind, that is attempt the same recovery as
 \fB-F\fR but read transactions otherwise deemed too old.
 .RE
 
+.sp
+.ne 2
+.na
+\fB-V\fR
+.ad
+.sp .6
+.RS 4n
+Attempt a verbatim import. This mimics the behavior of the kernel when loading
+a pool from a cachefile.
+.RE
+
 .P
 Specifying a display option more than once enables verbosity for only that
 option, with more occurrences enabling more verbosity.