reject invalid attribute settings

note that this is a pedantic conformance issue and waste of code. it
only affects broken code or code that is probing for conformance.

reorganize pthread data structures and move the definitions to alltypes.h

this allows sys/types.h to provide the pthread types, as required by
POSIX. this design also facilitates forcing ABI-compatible sizes in
the arch-specific alltypes.h, while eliminating the need for
developers changing the internals of the pthread types to poke around
with arch-specific headers they may not be able to test.

initial check-in, version 0.5.0