Fix signed shift warning for nlmacros BITMASK
Fix signed shift warning for nlmacros BITMASK

Clang gives a warning for the BITMASK macro:

    Shifting a negative signed value is undefined [-Wshift-negative-value]

The value in question is the literal "~0", which defaults to being a
signed integer. It is clearly intended to be used as an unsigned
integer, though, but happens to have the correct behavior because the
compilers/hardware don't do anything too crazy with an overflowing
left-shift in the signed case, despite it being undefined behavior.

nestlabs/nlutilities,nestlabs/nlutilities,nestlabs/nlutilities