pkg SAT_SOLVER bugs

Hi,

I found some bugs in PKG with regard to the SAT_SOLVER environment 
variable. Please find patch attached :-)

Issues fixed:
1) No need to use hash table when generating SAT rules for external 
solver. Variables are already in a linear array. Fix encoding and 
decoding of SAT data.
2) Endless variable loop caused pkg to crash.
3) it->inverse was checked for non-zero, while it should actually be 
checked for -1 only. SAT rules produces were all negative.

How to verify:

make -C /usr/ports/math/picosat all install clean

env SAT_SOLVER=picosat pkg upgrade

--HPS