winbindd: More simplification of cm_open_connection()

winbindd: More simplification of cm_open_connection()

This basically moves the functionality to connect the socket to the currently
preferred DC to a new helper function connect_preferred_dc() that is called from
the renamed function find_new_dc().

find_dc() now either returns a connected to the preferred DC or a new DC until
all possible DCs are exhausted and cm_open_connection() can just call find_dc()
to get a connected socket and pass it to cm_prepare_connection().

While at it reorder the args of find_dc() and make the only real out arg "fd"
the last one.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>