isc_nm_*connect() always return via callback

isc_nm_*connect() always return via callback

The isc_nm_*connect() functions were refactored to always return the
connection status via the connect callback instead of sometimes returning
the hard failure directly (for example, when the socket could not be
created, or when the network manager was shutting down).

This commit changes the connect functions in all the network manager
modules, and also makes the necessary refactoring changes in places
where the connect functions are called.