pthreads: Reimplement pthread types using non polluting types.

Use __ implementation specific namespace and adjust libthread_xu.
* The pthread_addr_t type was specific only to libc_r, remove it.

pthreads: Reimplement pthread types using non polluting types.

Use __ implementation specific namespace and adjust libthread_xu.
* The pthread_addr_t type was specific only to libc_r, remove it.
* The pthread_startroutine_t typedef was never used since initial fork.
* The mutex member in struct pthread_once was for libc_r only, rename
it to __sparelibc_r void pointer to keep ABI compatibility.
* Mangle struct names using __type_s scheme to avoid collisions with
user defined types in 3rd-party software packages.
* Adjust partly publicly visible struct pthread_once members too.

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pthreads - Bypass third-party allocators for internal allocations

* Adjust libthread_xu to always use libc's malloc/free (__malloc,
__free), bypassing third party allocators.

* Fixes interactions

pthreads - Bypass third-party allocators for internal allocations

* Adjust libthread_xu to always use libc's malloc/free (__malloc,
__free), bypassing third party allocators.

* Fixes interactions between third party alocators and pthreads
which often cause deadlocks or other chicken-and-egg issues,
or require serious hacks to work around.

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Add missing restrict qualifiers in POSIX function definitions/declarations.

This creates no further -Wrestrict warnings with gcc80 in buildworld.

pthread: Prefer explicit return on error.

Easier to follow the logic.

pthread: General pre-cleanup (style, typos etc)

No functional change.

WARNS level 4 cleanup.

Convert weak reference to strong reference so that static library
will work better.

Import initial version of 1:1 pthread library.