Fix awk usage

The zpool_id and zpool_layout helper scripts have been updated to
use the more common /usr/bin/awk symlink.  On Fedora/Redhat systems
there are both /bin/awk and /usr/bin/awk symlinks to your installed
version of awk.  On Debian/Ubuntu systems only the /usr/bin/awk
symlink exists.

Additionally, add the '\<' token to the beginning of the regex
pattern to prevent partial matches.  This pattern only appears to
work with gawk despite the mawk man page claiming to support this
extended regex.  Thus you will need to have gawk installed to use
these optional helper scripts.  A comment has been added to the
script to reflect this reality.

cmd/zpool_layout/zpool_layout

@@ -12,7 +12,7 @@
 # /etc/zfs/zdev.conf file, it allows the by-path naming convertion
 # to change and still keep the simple <channel><rank> naming.
 #
-AWK=${AWK:-/bin/awk}
+AWK=${AWK:-/usr/bin/awk}
 CONFIG=${CONFIG:-/etc/zfs/zdev.conf}
 BUSES=( 01 02 03 )
 PORTS=( 4 0 )