Generalize levelFromUid support.

Introduce a levelFrom=none|app|user|all syntax for specifying
per-app, per-user, or per-combination level assignment.
levelFromUid=true|false remains valid syntax but is deprecated.
levelFromUid=true is equivalent to levelFrom=app.

Update check_seapp to accept the new syntax.
Update seapp_contexts to document the new syntax and switch
from levelFromUid=true to levelFrom=app.  No change in behavior.

Change-Id: Ibaddeed9bc3e2586d524efc2f1faa5ce65dea470
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>