Fix 'zfs allow' for create time permissions

Fix 'zfs allow' for create time permissions

When no permission set is defined for a dataset the create time
permissions are incorrectly shown as if they were a permission set.
This change simply correct how allow permissions are displayed.

This commit also fixes a small manpage formatting issue and adds the
"zfs_allow_003_pos" test case to the ZFS Test Suite.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7519
Closes #7860