config: Add check_options helper

The purpose of this helper function is to perform actions common to
different interfaces, after all CRIU options have been parsed. This can
be used, for instance, to verify that a specific option was passed to
CRIU via CLI, RPC or config file.

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>