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 schedul

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

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