Require libblkid

Historically libblkid support was detected as part of configure
and optionally enabled.  This was done because at the time support
for detecting ZFS pool vdevs had just be added to libblkid and
those updated packages were not yet part of many distributions.
This is no longer the case and any reasonably current distribution
will ship a version of libblkid which can detect ZFS pool vdevs.

This patch makes libblkid mandatory at build time and libblkid
the preferred method of scanning for ZFS pools.  For distributions
which include a modern version of libblkid there is no change in
behavior.  Explicitly scanning the default search paths is still
supported and can be enabled with the '-s' command line option.

Additionally making libblkid mandatory means that the 'zpool create'
command can reliably detect if a specified device has an existing
non-ZFS filesystem (ext4, xfs) and print a warning.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2448

man/man8/zpool.8

@@ -83,13 +83,14 @@ zpool \- configures ZFS storage pools
 .LP
 .nf
 \fBzpool import\fR [\fB-o \fImntopts\fR\fR] [\fB-o\fR \fIproperty=value\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR]
-     [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-N\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR] [\fB-X\fR\] [\fB-T\fR\]] \fB-a\fR
+     [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-N\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR] [\fB-X\fR\] [\fB-T\fR\]] [\fB-s\fR] \fB-a\fR
 .fi
 
 .LP
 .nf
 \fBzpool import\fR [\fB-o \fImntopts\fR\fR] [\fB-o\fR \fIproperty=value\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR]
-     [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR] [\fB-X\fR] [\fB-T\fR\]] [\fB-t\fR]] \fIpool\fR |\fIid\fR [\fInewpool\fR]
+     [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR] [\fB-X\fR] [\fB-T\fR\]] [\fB-t\fR]] [\fB-s\fR]
+     \fIpool\fR | \fIid\fR [\fInewpool\fR]
 .fi
 
 .LP
@@ -1322,7 +1323,7 @@ Lists destroyed pools only.
 .ne 2
 .mk
 .na
-\fB\fBzpool import\fR [\fB-o\fR \fImntopts\fR] [ \fB-o\fR \fIproperty\fR=\fIvalue\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR] [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-N\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR]] \fB-a\fR\fR
+\fB\fBzpool import\fR [\fB-o\fR \fImntopts\fR] [ \fB-o\fR \fIproperty\fR=\fIvalue\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR] [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-N\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR]] [\fB-s\fR] \fB-a\fR\fR
 .ad
 .sp .6
 .RS 4n
@@ -1477,13 +1478,24 @@ Specify the txg to use for rollback.  Implies \fB-FX\fR. For more details about
 \fBWARNING\fR: This option can be extremely hazardous to the health of your pool and should only be used as a last resort.
 .RE
 
+.sp
+.ne 2
+.mk
+.na
+\fB\fB-s\fR
+.ad
+.RS 21n
+.rt
+Scan using the default search path, the libblkid cache will not be consulted.  A custom search path may be specified by setting the \fBZPOOL_IMPORT_PATH\fR environment variable.
+.RE
+
 .RE
 
 .sp
 .ne 2
 .mk
 .na
-\fB\fBzpool import\fR [\fB-o\fR \fImntopts\fR] [ \fB-o\fR \fIproperty\fR=\fIvalue\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR] [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR]] [\fB-t\fR]] \fIpool\fR | \fIid\fR [\fInewpool\fR]\fR
+\fB\fBzpool import\fR [\fB-o\fR \fImntopts\fR] [ \fB-o\fR \fIproperty\fR=\fIvalue\fR] ... [\fB-d\fR \fIdir\fR | \fB-c\fR \fIcachefile\fR] [\fB-D\fR] [\fB-f\fR] [\fB-m\fR] [\fB-R\fR \fIroot\fR] [\fB-F\fR [\fB-n\fR]] [\fB-t\fR]] [\fB-s\fR] \fIpool\fR | \fIid\fR [\fInewpool\fR]\fR
 .ad
 .sp .6
 .RS 4n
@@ -1635,6 +1647,17 @@ Used with "\fBnewpool\fR". Specifies that "\fBnewpool\fR" is temporary. Temporar
 Allows a pool to import when there is a missing log device.
 .RE
 
+.sp
+.ne 2
+.mk
+.na
+\fB\fB-s\fR
+.ad
+.sp .6
+.RS 4n
+Scan using the default search path, the libblkid cache will not be consulted.  A custom search path may be specified by setting the \fBZPOOL_IMPORT_PATH\fR environment variable.
+.RE
+
 .RE
 
 .sp