Follow changes in the pwmc(4) driver in relation to device filenames.

The driver now names its cdev nodes pwmcX.Y where X is unit number and
Y is the channel within that unit. Change the default device name from
pwmc0 to pwmc0.0. The driver now puts cdev files and label aliases in
the /dev/pwm directory, so allow the user to provide unqualified names
with -f and automatically prepend the /dev/pwm part for them.

Update the examples in the manpage to show the new device name format
and location within /dev/pwm.