Add some missing errno values to thr_new(2) and pthread_create(3).
In particular, EDEADLK was not documented in the latter.
While I'm here, improve some English and cross-references.
Reviewed by: kib
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D6663
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.
