#63 .xy files: Support both mm and mil units

[chrysn]

pcb can export .xy files both in mil and mm, and explains the units in
the header. This patch scans for the unit declaration, and stores it as
default unit for the following unit-less numbers.

[chrysn]

also, this fixes access to uninitialized memory in pick_and_place_get_float_unit.

[Sergey Alyoshin]

Acceped as git commit 90530171873e5fe6034479d0834915cd75ab1316