Make the flag properties use '< 0'.

When interpreting the truth value of an integer in a SIMD value, use the
'< 0' convention.
Clarify comments and code.

More firmly establish the convention of converting from boolean to
integer.
Elementwise select.

This renames the existing bitwise select to bitselect, and introduces a
new elementwise select, following the convention for boolean values in
SIMD of examining the sign bit.
Simplify signMask implementations.
Merge pull request #86 from sunfishcode/int32-to-bool

Boolean conventions