Add utility function _lp_bool

_lp_bool converts a return code (default to $?) into a "true"/"false"
value.
Fix #304: _lp_jobcount_color() when is tmux not installed

Refactor _lp_jobcount_color() to avoid running either 'screen' or 'tmux'
if they are not installed.
More _lp_jobcount_color refactoring