MFC r285715:

MFC r285715:

  Add an API for easily creating userspace threads in kernelspace.

  This change refactors the existing create_thread() function to be more
  generic. It replaces almost all of its arguments by a callback that can
  be used to extract the thread ID and copy it out to the right place, but
  also to perform additional initialization steps, such as setting the
  trapframe. This also makes the difference between thr_new() and
  thr_create() more clear in my opinion.

  This function is going to be used by the CloudABI compatibility layer.

  It looks like the OpenSolaris compatibility framework already provides a
  function called thread_create(). Rename this function to
  do_thread_create() and use a macro to deal with the namespacing
  conflict. A similar approach is already used for thread_exit().

git-svn-id: svn://svn.freebsd.org/base/stable/10@286843 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f