Add libzutil for libzfs or libzpool consumers

Adds a libzutil for utility functions that are common to libzfs and
libzpool consumers (most of what was in libzfs_import.c).  This
removes the need for utilities to link against both libzpool and
libzfs.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #8050

cmd/mount_zfs/mount_zfs.c

@@ -31,6 +31,7 @@
 #include <sys/mntent.h>
 #include <sys/stat.h>
 #include <libzfs.h>
+#include <libzutil.h>
 #include <locale.h>
 #include <getopt.h>
 #include <fcntl.h>