Make option parsing case-insensitive

[Steffan Sølvsten]

With the qbf benchmark I added an ascii_tolower function. One usecase is to make the -o option case-insensitive.

• Move the ascii_tolower from src/qbf.cpp to src/common.h
• Use this function in src/picotrav.cpp
• Use this function in src/knights_tour.cpp