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
This patch implements a service which provides the contents of a tar
archive via the 'File_system::Session' interface.
Configuration:
<config>
<archive name="tar_archive.tar" />
<policy label="label_of_client" root="/rootdir/for/client" />
</config>
Fixes#333.
