replace all remaining internal uses of pthread_self with __pthread_self

prior to version 1.1.0, the difference between pthread_self (the
public function) and __pthread_self (the internal macro or inline
function) was that the former would lazily initialize the thread
pointer if it was not already initialized, whereas the latter would
crash in this case. since lazy initialization is no longer supported,
use of pthread_self no longer makes sense; it simply generates larger,
slower code.

handle pending cancellation when enabling async cancellation

this is not strictly required by the standard, but without it, there
is a race condition where cancellation arriving just before async
cancellation is enabled might not be acted upon. it is impossible for
a conforming application to work around this race condition since
calling pthread_testcancel after setting async cancellation mode is
not allowed (pthread_testcancel is not specified to be
async-cancel-safe). thus the implementation should be responsible for
eliminating the race, from a quality-of-implementation standpoint.

optimize cancellation enable/disable code

the goal is to be able to use pthread_setcancelstate internally in
the implementation, whenever a function might want to use functions
which are cancellation points but avoid becoming a cancellation point
itself. i could have just used a separate internal function for
temporarily inhibiting cancellation, but the solution in this commit
is better because (1) it's one less implementation-specific detail in
functions that need to use it, and (2) application code can also get
the same benefit.

previously, pthread_setcancelstate dependend on pthread_self, which
would pull in unwanted thread setup overhead for non-threaded
programs. now, it temporarily stores the state in the global libc
struct if threads have not been initialized, and later moves it if
needed. this way we can instead use __pthread_self, which has no
dependencies and assumes that the thread register is already valid.

initial check-in, version 0.5.0