gEDA-dev: Checking for mm vs. mil display modes in PCB

[DJ Delorie]

There's no flag to say what mode the grid was defined in, there is a
flag for the *current* mode though.  You can set the grid to 0.5mm and
manually switch to mils, and the grid appears to be an unusual number.

What we'd need is a separate flag, part of the grid, that says which
units were last used (mm, mil, pcb) to set the grid.  We'd still need
the guessing code for things like loading the pcb, or when we set the
grid without units, but once set you can at least keep the *same*
units when incrementing the grid.