Add a restorecon_recursive built-in command to init.

Functionally equivalent to the restorecon -R toolbox command.
A use case is given by:
I48eaa2b9901ac8c978192c14493ba1058a089423

Also, fix error handling and documentation for restorecon command.

Change-Id: Ia7fbcc82645baf52c6bff0490d3492f458881cbb
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
(cherry picked from commit eec96b6d6829bcbcd451ea0a7104bb99e29d873e)