Initialize td_sel in the thread_init(). Struct thread is not zeroed
on the initial allocation, but seltdinit() assumes that td_sel is NULL
or a valid pointer. Note that thread_fini()/seltdfini() also relies
on this, but correctly resets td_sel to NULL.

Submitted by: luke.tw@gmail.com
PR: 199518
MFC after: 1 week