Now, we support thread trees which are spanning more than one level depth. Any
thread can create any number of threads provided that they are under the limits
which are enforced by the kernel.
Also with this commit, we have almost finished supporting SHARED space thread
creation in which stack and utcb space are allocated statically.
The difference between this thread library and the existing ones like pthreads
is the necessity of informing the library about the address range of the stack
and the l4 specific utcb. Utcb has not been supported yet. As for stack, there
is also a drawback: library does not support mapping. In other words, the stack
area in question has to be already mapped-in. Thus, for now we only support
threads sharing their address spaces: TC_SHARE_SPACE. In this respect, it is
very similar to pthreads.
