Fix althrd_sleep return value
Bump _WIN32_WINNT to 0x0502 (XP SP2) for GetThreadId
Make sure the duration given to althrd_sleep is valid
Remove almtx_normal and almtx_errorcheck
Fix some almtx_ return values
Rename althread_key_ wrappers to altss_ and move it to threads.h/c
Don't use DllMain to call altss destructors
Keep TlsDestructors within threads.c

This basically makes the threads implementation self-contained in threads.c and
threads.h, except for the UIntMap/RWLock implementations.
Fix a signed-unsigned comparison warning and clarify a comment
Make and use a C11-like altimespec_get wrapper function
Don't inline al_nssleep