Fix up proc_realparent to always return correct process.

Prior to the change it would always return initproc for non-traced processes.

This fixes ps apparently always returning 1 as ppid.

Pointy hat:	mjg
Reported by:	many
MFC after:	1 week
Partially revert r270964. Don't test for C++11 to define _Thread_local.

In addition to Clang 3.3, it turns out that GCC 4.7 in Ports also does
not support the _Thread_local keyword. Let's document this in a bit more
detail.

Reported by:	antoine@