libpthread: Move namespacing include to top of .c files.

Stuff like libc's namespace.h, or atomic_op_namespace.h, which does
namespacing tricks like `#define atomic_cas_uint _atomic_cas_uint',
has t

libpthread: Move namespacing include to top of .c files.

Stuff like libc's namespace.h, or atomic_op_namespace.h, which does
namespacing tricks like `#define atomic_cas_uint _atomic_cas_uint',
has to go at the top of each .c file. If it goes in the middle, it
might be too late to affect the declarations, and result in compile
errors.

I tripped over this by including <sys/atomic.h> in mips
<machine/lock.h>.

(Maybe we should create a new pthread_namespace.h file for the
purpose, but this'll do for now.)

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include correct header for "last minute just-in-case defensive addition
that's too trivial to check"

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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