MFC r275616:
Thread waiting for the vfork(2)-ed child to exec or exit, must allow
for the suspension.

MFC r275683 (by andreast):
Fix build for powerpc(32|64) kernels.

MFC r275686 (by andreast):
Fix kernel build for booke.

r275639 (by andrew) is not merged, since arm/arm/syscall.c is not
present on the stable/10 branch, and arm/arm/trap.c already includes
sys/kernel.h.
MFC r275617:
Do some refactoring and minor cleanups of the thread_single() code.
MFC r275618:
Check for stop condition in nfsd threads.
MFC r275619:
Check for bo_bufobj->bo_object for NULL and cache the value in local
variable to avoid NULL dereference in getnewbuf_reuse_bp(). The vnode
owning the buffer is not locked there.