ppp: Extend uci datamodel with persistency sypport

ppp: Extend uci datamodel with persistency sypport

PPP daemon can be put into persist mode meaning the
daemon will not exit after a connection gets terminated
but will instead try to reopen the connection.
The re-initiation after the link has been terminated
can be controlled via holdoff; this is helpfull in
scenarios where a BRAS is in denial of service mode
due to link setup requests after a BRAS has gone down

Following uci parameters have been added :
persist (boolean) : Puts the ppp daemon in persist mode
maxfail (integer) : Number of consecutive fail attempts which
puts the PPP daemon in exit mode
holdoff (interget) : Specifies how many seconds to wait
before re-initiating link setup after it has been terminated

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>