update to the latest netmap snapshot.
This includes the following:
- use separate memory regions for VALE ports
- locking fixes
- some simplifications in the NIC-specific routines
- performance improvements for the VALE switch
- some new features in the pkt-gen test program
- documentation updates

There are small API changes that require programs to be recompiled
(NETMAP_API has been bumped so you will detect old binaries at runtime).

In particular:
- struct netmap_slot now is 16 bytes to support an extra pointer,
  which may save one data copy when using VALE ports or VMs;
- the struct netmap_if has two extra fields;

MFC after:	3 days