[sysaction] rework to be more secure and helpful

setuid is active only for the time when action is executed. Exceptions
are used for unified error handling.
Also help is shown if needed.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Merge pull request #10 from sailfishos/improve-sysaction

[sysaction] rework to be more secure and helpful