Metadata extractor for SELinux Labels

[dstiliadis]

Requested behavior

The Trireme example can associate with the Trireme identity the SELInux labels of the
process that is being executed. This will help us to define SELinux based network
policies that span hosts, similar to what SELinux was trying to achieve with much
more complex implementations.

Expected behavior

When a process is instantiated through Trireme, the SELinux labels associated with
the process are captured and become part of the identity.

I can then use these SELinux labels to define a policy. For example if a file
has the labels: system_u:object_r:user_a these labels can become part of an authorization
policy.