thr_*(2): Add xrefs to what libthr implements using each syscall.

thr_*(2): Add xrefs to what libthr implements using each syscall.

Add text to thr_exit(2) and thr_new(2) discouraging their use in
applications since calling these in a process with libthr loaded will
confuse libthr and is likely to cause hangs or crashes.

The thr_kill2(2) call is not used by libthr and may be useful in special
applications.

The other calls can be used in applications but it should not be necessary.