sched: Use an integer for thread ids

Linux uses a 32-bit integer for pid_t, so let's do it as well. This is because
there are function in which we have to return our id back to the application.
One application is gettid, that we already have in the tree.

Theoretically, we could come up with a mapping between our 64-bit id and the
Linux one, but since we have to maintain the mapping anyway, we might as well
just use the Linux pids as our default IDs. The max size for that is 32-bit. It
is not enough if we're just allocating pids by bumping the counter, but again,
since we will have to maintain the bitmaps, 32-bit will allow us as much as 4
billion PIDs.

avi: remove unneeded #include

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>