os_thread_get_curr_id() et al. introduce unnecessary function call overhead

The os0thread.cc module unnecessarily defines non-inlined function call wrappers for trivial operations.

os_thread_pf() simply casts the argument to a different type, and it should not exist at all.

The functions os_thread_eq(), os_thread_yield(), os_thread_get_curr_id() are better defined as macros that refer to the Microsoft Windows and POSIX interfaces.

Because os_thread_get_curr_id() is being invoked in locking code, this simplification could slightly improve performance.