Fix a couple of mdoc issues in libusb.3 and usbdi.9.
Sync ehci.4, ohci.4 and uhci.4 with FreeBSD. Bring in xhci.4.
ehci.4: Break a long line.
tools - Fix return code in hammer-backup.sh

- When requesting help with '-h', exit with return code 0.

Submitted-by: Freddy DISSAUX
tools - Fix operator in hammer-backup.sh

Submitted-by: Freddy DISSAUX
sbin/hammer - Add 'header' option for mirror-dump directive.

- Provide means to dump PFS information only via 'mirror-dump'
  directive.
- Snapshots directory is not shown at the moment.

Submitted-by: YONETANI Tomokazu <y0netan1@dragonflybsd.org>
DragonFly-bug: <http://bugs.dragonflybsd.org/issues/1497>

This commit closes #1497