usr.sbin/pwm/pwm add support for flags

The pwm utility cant set the only flag defined (PWM_POLARITY_INVERTED) so this
patch add the option -I (capital letter i) to send it to the drivers.

None of existing PWM driver have implemented support for flags.
But soon:ish I will put up an review of a pwm driver using TI OMAP DMTimer.

Differential Revision: https://reviews.freebsd.org/D29137
MFC after: 2 weeks