Switch API reorganization.

Created a space for switch API and its implementations in userspace/switch and moved all switch API related files there. Changed the following in sw_api.h:
- removed functions declarations
- added parameters to the switch operations for more readability
- removed vlan_add_port and vlan_del_port because they are similar to if_add_trunk_vlans
- commented some functions in order to clarify the meaning of the returned values