Integration of socket activation for services

Integration of socket activation for services

The services provided using the option --ws-server
can now be linked to socket activation of systemd.

When afb-daemon is launched, the api to be provided
by socket activation is declared by adding the
option

   --ws-server=sd:APINAME

Where APINAME is the name of the provided api.

The binder looks to the prefix "sd:" to automatically
use the "listen fd" of name APINAME for serving
the api APINAME.

In the socket activation unit the connection
to this socket must be named using the
directive FileDescriptorName as below:

[socket]
FileDescriptorName=APINAME
...

Change-Id: I281e1a2b9fed3eac3bd0ee27b7f56df99df7bbd6
Signed-off-by: José Bollo <jose.bollo@iot.bzh>