fix readdir not to set ENOENT when directory is removed while reading

per POSIX, ENOENT is reserved for invalid stream position; it is an
optional error and would only happen if the application performs
invalid seeks on the underlying file descriptor. however, linux's
getdents syscall also returns ENOENT if the directory was removed
between the time it was opened and the time of the read. we need to
catch this case and remap it to simple end-of-file condition (null
pointer return value like an error, but no change to errno). this
issue reportedly affects GNU make in certain corner cases.

rather than backing up and restoring errno, I've just changed the
syscall to be made in a way that doesn't affect errno (via an inline
syscall rather than a call to the __getdents function). the latter
still exists for the purpose of providing the public getdents alias
which sets errno.

include cleanups: remove unused headers and add feature test macros

ditch the priority inheritance locks; use malloc's version of lock

i did some testing trying to switch malloc to use the new internal
lock with priority inheritance, and my malloc contention test got
20-100 times slower. if priority inheritance futexes are this slow,
it's simply too high a price to pay for avoiding priority inversion.
maybe we can consider them somewhere down the road once the kernel
folks get their act together on this (and perferably don't link it to
glibc's inefficient lock API)...

as such, i've switch __lock to use malloc's implementation of
lightweight locks, and updated all the users of the code to use an
array with a waiter count for their locks. this should give optimal
performance in the vast majority of cases, and it's simple.

malloc is still using its own internal copy of the lock code because
it seems to yield measurably better performance with -O3 when it's
inlined (20% or more difference in the contention stress test).

initial check-in, version 0.5.0