nova: request native ec cap via pager

Prerequisite to get rid of the base-nova special native_cap method in
cpu_session.

Vancouver/Seoul bootstrap code gets also much simpler.

Issue #478
nova: remove native_cap method from cpu_session

Remove native_cap method which caused the cpu_session implementation of the
gdb server to fail.

Issue #478
nova: fix thread in page fault and getting paused

If a thread caused a page fault and later on get be paused, then it left
the recall handler immediately due to the pause call instead of staying
in this handler.

Add some (complicated) state machine to detect and handle the case. Still not
waterproof, especially server threads may never get recalled if they never get
a IPC from the outside.

Fixes #478
nova: create sm for signalling via pager

The 'pause' call on base-nova assumes that a thread can solely block in its
associated semaphore. Main reason is that so core can unblock a thread in order
that the recall exception gets delivered and the register state can be
obtained.

Unfortunately the signal session implementation creates a semaphore, which is
unknown by the pager code. Instead create the semaphore via the pager of the
thread, so that the pager can unblock the signal thread when a pause is issued.

Issue #478
run: use gdb_monitor by autopilot

Issue #478