lsdev device name section headers should be printed by dv_print callback.

lsdev command does walk over devsw list, prints list element name and
will use dv_print() callback to print the device list.
Unfortunately this approach will add unneeded noise when there are no
particular devices detected.

To remove "empty" device section headers, the dv_print() callback
should print the header instead.

In addition, fixed dv_print callback for md module.

Reviewed by: imp
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D8551