Whitespace fixes
Omit child-name prefix in <if-args> check

When matching the 'label' session argument using '<if-args>' in a
routing table, we can omit the child name prefix because it is always
the same for all sessions originating from the child anyway. Therefore,
this patch adds a special case for matching session labels. It makes the
expression of label-specific routing more intuitive.
Unify 'Signal_dispatcher' interfaces

Several users of the signal API used custom convenience classes to
invoke signal-handling functions on the reception of incoming signals.
The 'Signal_dispatcher' pattern turned out to be particularly useful. To
avoid the duplication of this code across the code base, this patch
adds the interface to 'base/signal.h'.

Furthermore, the patch changes the 'Signal::num()' return type from int
to unsigned because negative numbers are meaningless here.

Fixes #511
Noux: delayed 'Child' object destruction

With this patch the destruction of Noux 'Child' objects gets delayed
further until the exit signal has been dispatched. This prevents the
self-destruction of the signal dispatcher, which is a member of the
'Child' object.

Fixes #603.
base: release signal context from signal list

Remove signal context object from signal source component list (_signal_queue)
before destruction, otherwise we get a dangling pointer.

On native hardware for base-nova, the signal source thread triggered page
faults in the Signal_source_component::wait_for_signal() method when the signal
context got freed up in Signal_session_component::free_context but was still
enqueued in Signal_source_component::_signal_queue.

Fixes #600
base: fix memmove implementation

Also make 'memcpy' to behave like memmove on region overlap.

Issue #604