Separate PID_MAX from BANG_MAX

This allows PID_FUNCTIONAL_RANGE to use a maximum duty cycle higher
than PID_MAX. This is useful for powerful heaters to heat quickly in
bang-bang mode, but use a lower duty cycle that is easier to stabilize
in PID mode.
Merge pull request #433 from whosawhatsis/bang-mode

Separate PID_MAX from BANG_MAX