MINOR: frontend: implement a dedicated actconn increment function

MINOR: frontend: implement a dedicated actconn increment function

When a new frontend connection is instantiated, actconn global counter
is incremented. If global maxconn value is reached, the connection is
cancelled. This ensures that system limit are under control.

Prior to this patch, the atomic check/increment operations were done
directly into listener_accept(). Move them in a dedicated function
increment_actconn() in frontend module. This will be useful when QUIC
connections will be counted in actconn counter.