nova: set pager and native ec cap for main thread

Prerequisite for follow up commit regarding base-nova specific Genode Spinlock
replacement.

Issue #986
nova: replacement of generic Genode spinlock

Turn Genode user-level spinlock into a user-level "helpinglock". This requires
support by the kernel introduced with kernel branch r5.

The commit avoids live-locks when multiple threads with SCs on different
priority levels compete for the same user-level Genode "spinlock".

Issue #986
nova: support usage of priorities in Genode/Nova

Enables finally the usage of priorities on base-nova.

Fixes #986