gsupplicant: Set the go intent to maximum when relevant

If being the master is requested, it will set the go intent to
maximum value.
peer_service: Handle registrations which specify to be the master

We internally keep a count of registered services which are set as
master. This will be then useful afterwards to set the GO intent from
the wifi plugin and gsupplicant, transparently.
peer_service: Add a function to tell if local peer is master or not

This will be used in the wifi plugin, and then gsupplicant, to set the
go intent relevantly. The function is declared in peer.h but the actual
implementation is in peer_service.h, I thought there were no need to
create yet another header file moreover there will not be much from
peer_service.c available for plugins. This function might be the only
one.
wifi: Add master parameter when connecting to a peer

This parameter tells if we want to be the GO or not. Thus it applies in
gsupplicant in a higher go intent.