Fix getmntpoint(3) to operate as it is documented in its manual page.

The -libutil function getmntpoint(3) is documented as accepting a device
name “with or without /dev/ prepended to it” but did not attempt to
prepend /dev/. This patch corrects the problem by prepending /dev/ to
names that do not begin with a '/'.

Reported-by: Dag-Erling Smørgrav
Differential Revision: https://reviews.freebsd.org/D53185
MFC-after: 1 week
Sponsored-by: Netflix