selinux: use different log priorites for log messages

selinux: use different log priorites for log messages

When selinux calls our callback with a log message, it specifies the
type as AVC or INFO/WARNING/ERROR. The question is how to map this to
audit types and/or log priorities. SELINUX_AVC maps to AUDIT_USER_AVC
reasonably, but for the other messages we have no idea, hence we use
AUDIT_USER_AVC for everything. When not using audit logging, we can
map those selinux levels to LOG_INFO/WARNING/ERROR etc.

Also update comment which was not valid anymore in light of journald
sucking in audit logs, and was actually wrong from the beginning —
libselinux uses the callback for everything, not just avcs.

This stemmed out of https://bugzilla.redhat.com/show_bug.cgi?id=1195330,
but does not solve it.