Constraints hook into the default validations, new constraints should derive from BaseConstraint.
This commit contains the code needed to add constraints to our model and a unittest for the UniqueConstraint option.

Add the following to a model field, to force uniqueness for a combination of keys:

<Constraints>
  <check001>                                                 <!--validation name, unique within this field-->
    <message>number should be unique</message>               <!--the message to output on validation failure -->
    <type>OPNsense\Base\Constraints\UniqueConstraint</type>  <!--the class to construct, derived from OPNsense\Base\Constraints\BaseConstraint -->
    <addFields>                                              <!--optional additional fields for the equation, defined in UniqueConstraint -->
       <field1>optfield</field1>
    </addFields>
  </check001>
</Constraints>

If there is not enough space in /tmp, it's really nothing we
can do about here.  This is not an essential file.

Taking the liberty to use this as an opportunity to kill the
usage of "e-mail".

Suggested by: @Antonio-Prado

Improve error messages for API calls

Example before: {"errorMessage":"Undefined offset: 0 [8]"}
Example after: {"errorMessage":"Error at \/usr\/local\/etc\/inc\/util.inc:952 - Undefined offset: 0 (errno=8)"}

While there, turn the probing upside down for general sanity:

o Only pull dmesg output once
o Pattern-match one pattern only
o Anchor the match to the line start

(get_interface_list) Properly handle an interface not appearing in dmesg

Also see: http://www.openssh.com/txt/release-7.0

The console would refuse to assign an IP that was used by the
PPTP or L2TP server (surprisingly not PPPoE).  This rule changed
during the MPD4 -> MDP5 migration, so remove that code.  Also,
the subnet check was never executed, good time to remove it as
well.

Spotted by: @fabianfrz

Reported by: @Antonio-Prado

(mvc) more work on migrations, fix autoloader, serialize to config when all steps for this model are finished

(mvc) BaseModel add getVersion() to retrieve the running config version, commit changes to in memory config after every migration step.

Errors will be send to syslog, current policy is to always upgrade and ignore migration errors.
(there's not really a way back, other then undo pkg install and revert the full config)

(Base) Default to inf rows not 9999