pthread: Fix some warnings, do not raise WARNS to 6.

Using -Os will create lots of -Winline, thus leave at 5 by default.
Sorry for __is_threaded.

pthread: Fix return value for pthread_attr_setschedpolicy()

There are to possible interpretations of ENOTSUP in POSIX specs.
After some research and trial and error testing for now stick to:
on unsu

pthread: Fix return value for pthread_attr_setschedpolicy()

There are to possible interpretations of ENOTSUP in POSIX specs.
After some research and trial and error testing for now stick to:
on unsupported [known] attribute ENOTSUP should be returned.
Check sched parameters explictly.

While there, rename too generic local variable 'local' and
move up the <sys/syscall.h> include.

show more ...

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.

Remove unused macro.

Import initial version of 1:1 pthread library.