archlinux: update PKGBUILD for netmap-2.0-2

Modifications:
    - netmap code downloaded by google public repository
    - netmap headers installed under /usr/include/net
pkt-gen: print # of ring slots in verbose mode
pkt-gen: use 2nd arg for initial nm_open()

4th arg was being ignored by nm_open() since ->self was NULL.
pkt-gen: removed global nmd_flags field