map interruption of close by signal to success rather than EINPROGRESS

commit 82dc1e2e783815e00a90cd3f681436a80d54a314 addressed the
resolution of Austin Group issue 529, which requires close to leave
the fd open when failing with EINTR, by returning the newly defined
error code EINPROGRESS. this turns out to be a bad idea, though, since
legacy applications not aware of the new specification are likely to
interpret any error from close except EINTR as a hard failure.

overhaul aio implementation for correctness

previously, aio operations were not tracked by file descriptor; each
operation was completely independent. this resulted in non-conforming
behavior for non-seekable/append-mode writes (which are required to be
ordered) and made it impossible to implement aio_cancel, which in turn
made closing file descriptors with outstanding aio operations unsafe.

the new implementation is significantly heavier (roughly twice the
size, and seems to be slightly slower) and presently aims mainly at
correctness, not performance.

most of the public interfaces have been moved into a single file,
aio.c, because there is little benefit to be had from splitting them.
whenever any aio functions are used, aio_cancel and the internal
queue lifetime management and fd-to-queue mapping code must be linked,
and these functions make up the bulk of the code size.

the close function's interaction with aio is implemented with weak
alias magic, to avoid pulling in heavy aio cancellation code in
programs that don't use aio, and the expensive cancellation path
(which includes signal blocking) is optimized out when there are no
active aio queues.

fix handling of EINTR during close()

austin group interpretation for defect #529
(http://austingroupbugs.net/view.php?id=529) tightens the
requirements on close such that, if it returns with EINTR, the file
descriptor must not be closed. the linux kernel developers vehemently
disagree with this, and will not change it. we catch and remap EINTR
to EINPROGRESS, which the standard allows close() to return when the
operation was not finished but the file descriptor has been closed.

remove bogus extra logic for close cancellability

like all other syscalls, close should return to the caller if and only
if it successfully performed its action. it is necessary that the
application be able to determine whether the close succeeded.

overhaul pthread cancellation

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

global cleanup to use the new syscall interface

initial check-in, version 0.5.0