Improve systemd script to not leave stale sharetab

The systemd script zfs-share.service does 'zfs share -a' to share
any required datasets. Unfortunately, /etc/dfs/sharetab is stale
from the previous boot. Delete it before we share.

Signed-off-by: Dan Swartzendruber <dswartz@druber.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2883