relix:  Define gettid() syscall number
Genie:  Add Process::gettid() that returns the pid
sys/wait.h:  Define __WTHREAD
Genie:  Wait for threads with __WTHREAD
Genie:  Don't process-terminate non-leader threads

Don't send SIGCHLD to the parent or orphan child processes when a thread
(other than the main thread) terminates.
Genie:  Pass separate tid argument to Process ctor
Merge branch 'new-Process-tid-arg' into thread-id
Merge branch 'define-gettid' into gettid-syscall
Genie:  Add gettid() system call
libc:  Define gettid() syscall glue
Genie:  Get tid instead of pid when appropriate

Call gettid() instead of GetPID() when we need the actual thread's index
in the "process" vector, rather than that of the thread leader.
Genie:  Ignore non-leader threads in waitpid()

When waitpid() is called to wait for a child matching some criterion (as
opposed to a specific process), don't consider a non-leader thread to be
a match.
Genie:  Store thread ID distinct from Process' pid
Merge branches 'Terminate-threads' and 'waitpid-WTHREAD' into kernel-threads
Merge branch 'thread-id' into kernel-threads
Genie:  Add NewThread()
Genie:  Allow CLONE_THREAD
relix:  Declare gettid() in relix headers
Merge branches 'kernel-threads' and 'gettid-syscall' into thread-support
Merge branch 'thread-support' into pthreads