dbghelp: Added infrastructure to compute variable location at runtime (as opposed to debug info parse time).

Use it to send the variables depending on not known frame register at parse time.
Made just a stub for the location computation function for dwarf2.

dbghelp: Use the location info structure thoughout the code to handle the location of a data variable.

Make use of it in dwarf.c to handle correctly variable location computation.
Split code to handle both location with parse time computation
(current code) and yet to come run-time computation

This makes the code simpler to read, a bit more efficient, and
furthermore it'll be needed in future patches.

The DC palette only matters for DIB_PAL_COLORS.

winepath: Fix --unix so it works whether the Windows path refers to an existing file/directory or not.