FreeBSD_HEAD_amd64_gcc - Build #1287 - Fixed

FreeBSD_HEAD_amd64_gcc - Build #1287 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1287/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1287/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1287/console

Change summaries:

301956 by hselasky:
Implement libusb_get_version() and update libusb manual page.

Approved by:	re (glebius)
Requested by:	swills
MFC after:	1 week

301955 by avg:
fix a zfs boot regression introduced in r300117 by accident

There is no reason to return non-zero value from zfs_probe_partition()
as that causes following partitions to not be probed for ZFS vdevs.
A particular scenario that I encountered is a GPT partitioned disk
where several partitions have freebsd-zfs type.  A partition with a lower
index is used as a cache (l2arc) vdev and in that case case zfs_probe()
returned a non-zero status.  That status was returned to ptable_iterate()
and caused it to abort the iteration.  Because of that the subsequent
partitions were not probed and a root pool was not discovered resulting
in a boot failure.

While there fix the style for nearby return statements.

Approved by:	re (kib)