Allow for arbitrary MI scheduler implementations.

A concrete result is enabling unpatched libpthread to run on the
rumprun stacks (e.g. Xen and bare metal) with a non-NetBSD scheduler.
Those schedulers hook into the existing _lwp_frobnitz() NetBSD syscall
interfaces (well, "syscall" interfaces in that scenario ;)

More specifically about the change itself:

1) instead of calling _lwp_makecontext() followed by _lwp_create()
  and passing the entry point in ucontext_t (MD) through the calls, roll
  the calls into pthread__makelwp() and allow alternate implementations
  for that MI interface.

2) allow compile-time overriding of __lwp_gettcb_fast() or
  __lwp_getprivate_fast, which are inline and leak MD scheduler/thread
  details into libpthread

Additionally, two small nits:

I)  define LIB=pthread before including mk.conf so that it's possible
    to test for LIB==pthread in mk.conf

II) make it possible to leave out pthread_cancelstub.c.  This is required
    by the current implementation of rumprun-posix (i.e. rumprun on
    POSIX hosts) due to symbol collisions.  It needs to be fixed properly
    some day, but for now allows an almost-correct libpthread to run.
    I am sure @justin will be happy to explain the details ;)

no change to NetBSD
tested: anita+atf

(pooka)