The aio_return() must be called regardless of the error returned by
aio_error().  Not calling it resulted in various problems up to segmentation
faults (as AIO events are level-triggered and were reported again and again).

Additionally, in "aio sendfile" case r->blocked was incremented in case of
error returned from ngx_file_aio_read(), thus causing request hangs.

Second argument (cpusetsize) is size in bytes, not in bits.  Previously
used constant 32 resulted in reading of uninitialized memory and caused
EINVAL to be returned on some Linux kernels.

FreeBSD kernel checks headers/trailers pointer against NULL, not
corresponding count.  Passing NULL if there are no headers/trailers
helps to avoid unneeded work in kernel, as well as unexpected 0 bytes
GIO in traces.

Requested by Igor Sysoev.

If process exited abnormally while holding lock on some shared memory zone -
unlock it.  It may be not safe thing to do (as crash with lock held may
result in corrupted shared memory structure, and other processes will
subsequently crash while trying to access shared data), therefore complain
loudly if unlock succeeds.

It is currently used from master process on abnormal worker termination to
unlock accept mutex (unlocking of accept mutex was broken in 1.0.2).  It is
expected to be used in the future to unlock other mutexes as well.

Shared mutex code was rewritten to make this possible in a safe way, i.e.
with a check if lock was actually held by the exited process.  We again use
pid to lock mutex, and use separate atomic variable for a count of processes
waiting in sem_wait().

A fix for the broken build applied.

Patch from Igor Sysoev.

for output of ./configure options, etc., since ngx_log_stderr() output
length is limited by 2048 characters defined as NGX_MAX_ERROR_STR.