Protection from stale write events in epoll.

Stale write event may happen if epoll_wait() reported both read and write
events, and processing of the read event closed descriptor.

Linux AIO related fixes:

*) Fixing Linux AIO syscalls return value handling:
syscall(2) uses usual libc convention, it returns -1 on error and
sets errno. Obsolete _syscall(2) returns negative value of error.

*) Fixing Linux AIO initiatialization: AIO operations are disabled
if kernel does not support them.  Previously worker just exited.

*) The "worker_aio_requests" directive.

The default value is 32 AIO simultaneous requests per worker. Previously
they were hardcoded to 1024, and it was too large, since Linux allocated
them early on io_setup(), but not on request itself. So with default value
of /proc/sys/fs/aio-max-nr equal to 65536 only 64 worker processes could
be run simultaneously. 32 AIO requests are enough for modern disks even if
server runs only 1 worker.

*) change ngx_time_update() interface

*) change ngx_time_update() interface since there are no notification methods
   those return time

since localtime_r() is not Async-Signal-Safe function

   header files
*) delete insignificant comments