Add a way to distinguish between forking and thread creation in schedtail.

For CloudABI we need to initialize the registers of new threads
differently based on whether the thread got created through a fork or
through simple thread creation.

Add a flag, TDP_FORKING, that is set by do_fork() and cleared by
fork_exit(). This can be tested against in schedtail.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D3973