init: add a restorecon_recursive built-in command to init.

Functionally equivalent to the restorecon -R toolbox command.

Also, fix error handling and documentation for restorecon command.

<3>[    5.447612] init: /init.rc: 230: invalid command 'restorecon_recursive'

Signed-off-by: Humberto Borba <humberos@gmail.com>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Change-Id: Ia7fbcc82645baf52c6bff0490d3492f458881cbb