add minimal NetBSD support

mostly ride on the existing FreeBSD support.

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
keep "kernel name" for each netdev

where interface renaming is not supported (NetBSD), remember both of
our netdev name and the correspoinding kernel name separately.
the latter is necessary to talk with kernel using interface names.
eg. ifioctls, bpf

XXX there should be a proper way to query kernel name.
Ben Pfaff suggested ovs-appctl but this patch doesn't implement it. (yet)

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
implement set_etheraddr for NetBSD

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
implement get_stats for NetBSD

Signed-off-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
Yamamoto-san patch 'implement get_next_hop for NetBSD'

'git am' failed due to INSTALL.userspace conflict; this is the remaining hunk.
netdev: Use netdev_bsd_get_next_hop on FreeBSD

It came from Yamamoto-san's NetBSD port and is applicable to FreeBSD with a
compatibility macro.
Use UINT64_MAX as required for unsupported stats
Refactor FreeBSD get_stats to allow merging
Refactor common parts of ...get_stats