Add a COMMENT describing what each component roughly does.

"make describe" prints the comment.

Requested/inspired by Vincent Schwarzer on rumpkernel-users

branches: 1.9.6;
Rename rumpcomp_user.c -> virtif_user.c
Convert to new-style RUMPCOMP_USER_SRCS in Makefile.

Separate common bits for all interfaces implementing virtif hypercalls
from Makefile to Makefile.virtif. The latter is meant to be .included.

Determine the interface name and the names of exported symbols based
on VIRTIF_BASE. This allows using various different packet-shoveling
backends in the same rump kernel.

branches: 1.6.6;
Rerevert to preprevious now that this builds with build.sh

Guess the RUMPCOMP_USER stuff wasn't ready to be in the NetBSD tree yet,
so revert previous commits to fix build. I'll look at rereverting when
toolchain/47644 is fixed and clean buildtests are again possible.

Push the bits accessing the networking backend into hypercalls.
This allows a few highly desirable things to happen:

1) Linux support, which is included in this commit (thanks to wanq for
sending me the magic bits for accessing /dev/net/tun)
2) Reusing the kernel-side of the interface for devices such as
hardware packet processors which interact directly with user vm spaces
(not included in the commit).

branches: 1.3.12; 1.3.22;
Use a bunch of weak symbols to determine which network components
are present. This works in userspace as opposed relying in link
sets, which fail miserably. Later, when the networking stack
becomes modularized, we can move to a dynamic scheme like with file
systems.

Also, this change allows us to do proper autoconfig, namely attach
the loopback interface iff it is present.

branches: 1.2.2; 1.2.10; 1.2.12; 1.2.16;
kill default DBG=-g

branches: 1.1.2;
Implement a virtual userspace networking interface. This is required
for any sensible operation when running the entire TCP/IP stack in
userspace (as opposed to libsockin which uses host kernel networking
to provide PF_INET).

While this basically works (although it is quite barebones), it
depends on some bit of cleanup in librump and is not built by
default yet.