#409 standardize handling of [DISPLAY]POSITION_OFFSET in our guis

[Sebastian Kuzminsky]

These docs say it can have the value RELATIVE or MACHINE:

http://www.linuxcnc.org/docs/2.6/html/config/ini_config.html#sub:DISPLAY-section

Some of our GUIs accept ABSOLUTE instead of MACHINE.

This ticket is to figure out whether the docs or the code are wrong (ie, which is better alternative to "RELATIVE"; "MACHINE" or "ABSOLUTE"?), and fix it as follows:

• In 2.6, guis should accept whatever they were accepting before, and also the correct one.

• In 2.7 they should accept the same things as 2.6, but complain if they get the wrong one.

• In master (aka 2.8) they should only accept the correct value (ie the documented one).

Also make sure stepconf and pncconf write the correct values in all three branches.