b696c61 Move get_units() to prefs.c...

based on: http://github.com/torvalds/subsurface/commit/b696c61389680b9ccf2582a445c9fd7a7e1da360

compiled with: mingw 3.4.5

notes:
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>