Group flag variables for neatness
Add new flag -build that allows multiple spec files in one invocation

For example:

    demand -build *

    demand -build -upgrade ~/bin/foo

    perl -ne 'close(ARGV) if $.>1; print "$ARGV\0" if m{^#!/usr/bin/env\s+demand$}' -- * \
    | xargs -0 --no-run-if-empty demand -build

The old -run flag was the negated version of this (you needed to
always pass -run=false), but without the possibility of passing
multiple spec files. It was clumsy to use, and is now removed.