ZFSROOT not hardcoded to zpool or dataset 'tank'

For ZFS root readonly case, the WHEREISROOT variable was grep-ing for 'tank', expecting a zpool with that name.

A workaround of naming a dataset 'tank', for example, 'sys/ROOT/tank' or 'sys/tank/pfSense' is compatible, however this shouldn't be hardcoded in.

My proposed change replaces WHEREISROOT with ZFSROOT using awk. I noticed awk is used elsewhere in this script. However it is still simple code that is not resilient against unexpected circumstances, let alone failure.
Merge pull request #1 from evansus/patch-1

ZFSROOT not hardcoded to zpool or dataset 'tank'
Use 'zfs mount' to get ZFSROOT dataset

using just zfs, grep, and cut, with a simple regexp
Merge pull request #2 from evansus/master

simpler zfs mount command
Merge pull request #602 from evansus/patch-1

ZFSROOT not hardcoded to zpool or dataset 'tank'
correct debug issue

Accidentally changed this from executing the zfs command to echoing the
expected command in previous commits.
This had been set while debugging a better command to set ZFSROOT
variable.
Merge pull request #608 from evansus/master

Merge commit to correct debugging issue
Fix indent and whitespaces
Fix and improve swap_usage()

- Fix a typo on awk syntax
- Use php functions to extract desired value from swapinfo output
Use exec directly instead of exec_command
Fix dnsmasq host overrides and dhcp integration

. Do not execute following actions when dnsmasq is disabled:
  . Add host overrides to /etc/hosts
  . Register DHCP leases in DNS Forwarder
  . Register DHCP static mappings in DNS forwarder

It should fix issue reported at following forum post:

http://forum.pfsense.org/index.php?topic=58871
Tidy up "firewall_nat.php" XHTML

Add "closehead" PHP variabled and add closing HEAD statement
Close INPUT, BR and IMG tags and add ALT to IMG tags
Tidy up "firewall_nat_edit,php" XHTML

Add "closehead" PHP variable
Add type to LINK
Close INPUT and BR tags
Update HTML Boolean operators
Add CDATA section to SCRIPT tags
Tidy up "firewall_nat_1to1.php" XHTML

Close BR and IMG tags and add ALT to IMG tags
Move NOWRAP to class statement
Deprecate ampersand in ANCHOR tags
Replace opening TR tag with closing TR tag
Tidy up "firewall_nat_1to1_edit" XHTML

Add CDATA section to SCRIPT tag
Close BR and INPUT tags
Update HTML Boolean operators
Add missing closing TD tag
Tidy up "firewall_nat_ntp" XHTML

Close BR and IMG tags and add ALT to IMG tags
Deprecate ampersand in ANCHOR tags
Move NOWRAP to class statement
Tidy up "firewall_nat_out.php" XHTML

Close BR, INPUT and IMG tags and ALT to IMG tags
Update HTML Boolean operators
Move NOWRAP to class statement
Move CENTER tag to ALIGN statement in TD tag
Add missing closing SPAN tag
Add missing closing P tag
Tidy up "firewall_nat_out_edit.php" XHTML

Add CDATA section to SCRIPT tags"
Close INPUT and BR tags
Update HTML Boolean operators
Add missing opening TR tag
Tidy up "firewall_nat_ntp_edit.php" XHTML

Close BR and INPUT tags
Update HTML Boolean Operators
Add missing closing TD tag
Merge pull request #609 from ExolonDX/branch_01

Tidy up "firewall_nat.php" XHTML