MFC r196730:

MFC r196730:
Remove the altkstacks, instead instantiate threads with kernel stack
allocated with the right size from the start. For the thread that has
kernel stack cached, verify that requested stack size is equial to the
actual, and reallocate the stack if sizes differ.

Introduce separate kernel stack cache that keeps some limited amount of
preallocated kernel stacks to lower the latency of thread allocation.

Not a merge: instead of removing td_altkstack* members of struct thread,
replace them with placeholders to keep struct thread layout on the
stable branch.

Also, record r196640, r196644 and r196648 as merged.

Approved by:	re (kensmith)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196977 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f