fix fclose of permanent (stdin/out/err) streams

this fixes a bug reported by Nuno Gonçalves. previously, calling
fclose on stdin or stdout resulted in deadlock at exit time, since
__stdio_exit attempts to lock these streams to flush/seek them, and
has no easy way of knowing that they were closed.

conceptually, leaving a FILE stream locked on fclose is valid since,
in the abstract machine, it ceases to exist. but to satisfy the
implementation-internal assumption in __stdio_exit that it can access
these streams unconditionally, we need to unlock them.

it's also necessary that fclose leaves permanent streams in a state
where __stdio_exit will not attempt any further operations on them.
fortunately, the call to fflush already yields this property.

refactor stdio open file list handling, move it out of global libc struct

functions which open in-memory FILE stream variants all shared a tail
with __fdopen, adding the FILE structure to stdio's open file list.
replacing this common tail with a function call reduces code size and
duplication of logic. the list is also partially encapsulated now.

function signatures were chosen to facilitate tail call optimization
and reduce the need for additional accessor functions.

with these changes, static linked programs that do not use stdio no
longer have an open file list at all.

fix false ownership of stdio FILEs due to tid reuse

this is analogous commit fffc5cda10e0c5c910b40f7be0d4fa4e15bb3f48
which fixed the corresponding issue for mutexes.

the robust list can't be used here because the locks do not share a
common layout with mutexes. at some point it may make sense to simply
incorporate a mutex object into the FILE structure and use it, but
that would be a much more invasive change, and it doesn't mesh well
with the current design that uses a simpler code path for internal
locking and pulls in the recursive-mutex-like code when the flockfile
API is used explicitly.

fix more unused variable warnings

some of these were coming from stdio functions locking files without
unlocking them. I believe it's useful for this to throw a warning, so
I added a new macro that's self-documenting that the file will never
be unlocked to avoid the warning in the few places where it's wrong.

correct locking in stdio functions that tried to be lock-free

these functions must behave as if they obtain the lock via flockfile
to satisfy POSIX requirements. since another thread can provably hold
the lock when they are called, they must wait to obtain the lock
before they can return, even if the correct return value could be
obtained without locking. in the case of fclose and freopen, failure
to do so could cause correct (albeit obscure) programs to crash or
otherwise misbehave; in the case of feof, ferror, and fwide, failure
to obtain the lock could sometimes return incorrect results. in any
case, having these functions proceed and return while another thread
held the lock was wrong.

add bsd fgetln function

optimized to avoid allocation and return lines directly out of the
stream buffer whenever possible.

add proper fuxed-based locking for stdio

previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.

i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.

this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.

fix fclose return status logic, again

the previous fix was incorrect, as it would prevent f->close(f) from
being called if fflush(f) failed. i believe this was the original
motivation for using | rather than ||. so now let's just use a second
statement to constrain the order of function calls, and to back to
using |.

fix undefined call order in fclose, possible lost output depending on compiler

pcc turned up this bug by calling f->close(f) before fflush(f),
resulting in lost output and error on flush.

initial check-in, version 0.5.0