1:N threading

[Geoffrey C. Speicher]

OK, so we've got 1:N threading (libc_r), 1:1 threading (thr), and M:N
threading (KSE).  Each model has its own merit depending on the
application.

However, it would still be nice if the 1:N model didn't block the whole
process when a thread blocks.  Is there any reason to hold onto a pure
userland implementation of 1:N?  Can libc_r be implemented in terms of
KSE?

Geoff