Fix value returned by smack_label_length()

In some cases smack_label_length have returned 0 on incorrect label.
Now smack_label_length() always returns -1 in case of incorrect label.
(cherry picked from commit f7e4232a319b269f6214af660173eebaa605ad9e)
Use smack_label_length() internally instead strnlen()

Use smack_label_length() to check correctness labels instead strnlen().
(cherry picked from commit 6d3fd3049e08377f75b554cedcd200286fbf359c)
Fix man pages

Regression from 1edba54.
(cherry picked from commit e6f32e4403ad549102cfbdbd02b4dcd254da84d2)