Move network add/remove operations to a common function

Move network add/remove operations to a common function

All the 3 control interfaces: socket based, dbus & binder needs to
perform the same sequence of steps for network add/remove. So, move
these to a common utility method in |wpa_supplicant.c| instead of
duplicating the code everywhere.

Signed-off-by: Roshan Pius <rpius@google.com>