Update zfs-functions.in

Update zfs-functions.in

The init.d zfs-share script does not perform the intended
action without having a variable set for ZFS_SHARE and
ZFS_UNSHARE

Assign default values to ZFS_SHARE and ZFS_UNSHARE. Export
the environment variables after sourcing the configuration
file.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Signed-off-by: Allen Holl <allen.m.holl@gmail.com>
Closes #10341
Closes #10382