Fix handling of DNS names with '-' in them for sharenfs

An old FreeBSD bugzilla report PR#168158 notes that DNS
names with '-'s in them cannot be used for the sharenfs
property. This patch fixes the parsing of these DNS names.
The only negative affect this patch might have is that,
if a user has incorrectly separated options with a '-'
the sharenfs setting will no longer work once this patch
is applied.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Closes #16529