Teach rump how to process __link_set_sysctl_funcs so it can handle
modules the same as a real kernel.

Partly addresses PR kern/55088 - __link_set_evcnts not yet handled
(that will happen later)

branches: 1.3.28;
domains are attached by module(-like) constructors, so no need to
play link_set games with them.

branches: 1.2.2; 1.2.12; 1.2.22; 1.2.26; 1.2.36;
Introduce RUMP_COMPONENT. It behaves mostly like a simplified
module which is linked into the kernel and cannot be unloaded.
The main purpose is to get the proper constructors run and create
any /dev nodes necessary for said component. Once more of the
kernel (e.g. networking stack and device drivers) are converted to
MODULE and devfs pops up from somewhere, rump components can be
retired.

branches: 1.1.2;
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).