Thread: remove globals. Thread create must succeed

Thread: remove globals. Thread create must succeed

Remove the references to global variables from Thread.h. They are really
unecessary. In every case, the printout is followed by an assert which
will deliver the exact same information.

Assert that thread creation succeeds. Nobody was checking the return
value of Thread::create() previously. Added a new function,
Thread::try_create(), which programmers can use if they do want to check
the value of Thread::create() and handle it appropriately.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>