Move rumpuser probes to a separate routine.

no functional change
Move librt probe to rumpuser probe bits.
rename rumpuser_probe to probe_rumpuserbits (consistency)
Probe for __thread only in !KERNONLY

The probe almost invariably went wrong if building for a non-POSIX env.
KERNONLY builds can still set RUMP_CURLWP manually, in case they want
something else than the default (currently "hypercall").
Add a probe_compiler routine and move related bits there.
move comment to correct location (and massage it a bit)
probe_rumpuserbits iff !${KERNONLY}
adjust lines to 80cols

no functional change intended
remove old compat whine (comment says to remove after 9/2014)
clean up some temporary files
Don't worry about native vs. xcompile in <linux/if_tun.h> probe.

Can't figure out why it should matter.
Make the linux/if_tun.h probe a rumpuserbits probe.