Put the kernelside rump kernel headers into <rump-sys> instead of
sprinkling them around the faction directories. Avoids having
to add a CPPFLAGS (or several) to pretty much every component
Makefile.

Leave compat headers around in the old locations.

The commit changes some autogenerated files, but I'll fix the
generators shortly and regen.

Add a COMMENT describing what each component roughly does.

"make describe" prints the comment.

Requested/inspired by Vincent Schwarzer on rumpkernel-users

branches: 1.8.6;
rename component.c -> sockin_component.c

rename rumpcomp_user.* -> sockin_user.*

branches: 1.6.6;
Move the rumpuser_net set of hypercalls to be a private to the sockin
component. This cleans up the generic hypercall interfaces from ones
specific to only one component. They should always have been private,
but the infrastructure to "make it so" didn't exist earlier.

no functional change

branches: 1.5.12; 1.5.22;
Use linker script to make __start/stop_link_set_modules be present
in libs built with binutils >=2.19. This is a less error-prone
method than the previous where components had to be tagged in the
Makefile as modules (and if they weren't, things broke. and vice
versa).

binutils 2.19 has changed the old behaviour of defining __start_SECTNAME
for orphaned sections to using PROVIDE. What this means is that
unless a rump component internally references that symbol, it will
not be included in the component shared library, and hence cannot
be referenced when the component is loaded. Add a workaround which
works both with 2.16 and 2.19: force a reference to the __start
symbol internally and hence retain it in the resulting library.

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; 1.2.4;
Convert compile-time SOCKIN_NOTHREADS into runtime rump_threads check.

branches: 1.1.2; 1.1.4; 1.1.6; 1.1.8;
Add a very simplistic PF_INET/SOCK_DGRAM domain/proto implementation,
which delegates the work to host kernel sockets. This does not
run the entire kernel TCP/IP stack in userspace and therefore does
not require the ability to send or receive raw packets. This
implies that root priviledges are not required. As already said
above, only supports UDPv4 for now. Extending should be easy.