hw: adjust SIGNAL SLAB sizes to new conditions

fix #950
hw: missing initialization in signal handler

fix #951
x86emu: avoid compiler warning
base: call deconstructor of context area

Issue #905
base: count caps replied by a rpc function

Issue #905
nova: support cap receive window of various sizes

Open a capability receive window according to the number of the capabilities
expected as out parameter per RPC function.

Typically the number of capabilities expected during the reply of a RPC/IPC
call is 0 to 1. Before this patch ever a capability receive window of 4 has
been opened.

On Nova the capability selectors of receive windows must be naturally aligned
to the size/order of the expected capabilities. This leads until now to the
issue that the left over 3 capabilities couldn't be reused for new IPCs since
they are not naturally aligned to 4.

Issue #905
nova: introduce cap_map

to be used for ref counting of Native_capabilities

* similar as used for base-foc
* will replace nova specific cap_sel_allocator interface

Issue #905
nova: initialize cap_map

Issue #905
nova: add inc/dec to native_types.h

Issue #905
nova: use cap_map for IPC

Issue #905
nova: remove cap_sel - use cap_map

Fixes #905
use 'const &' for Native_capability where possible

Native_capability are smart pointer and introduce some overhead when they
get copied for base-foc and base-nova. Reduce the performance penalty by
using const ref where possible.

Issue #905