natacontrol(8): Add commands to tweak power/acoustic management features.

Sometimes disks designed to operate in low power mode by default
constantly spin up/down during operation which leads to reduced
performance.

This commit adds the 'feature' command to natacontrol(8) which
can be used to tweak the 'advanced power management' and 'automatic
acoustic management' features. Their current setting could already
be viewed before this commit using the 'natacontrol cap' command.

I'd like this to be possible with camcontrol(8) too for CAM attached
disks, but afaict these are ATA specific features so I guess we'd
have to integrate ATA into CAM too before that is possible.

Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2531>
Reported-by:   Maurizio Lombardi <m.lombardi85@gmail.com>
Taken-from:    FreeBSD PR 81692
Tested-by:     Maurizio Lombardi <m.lombardi85@gmail.com>