Signed-off-by: Piotr Sikora <piotrsikora@google.com>

It is more effective, because it doesn't require a separate lseek().

No functional changes.

This is in addition to 6fce16b1fc10.

This fixes suboptimal behavior caused by surplus lseek() for sequential writes
on systems without pwrite().  A consecutive read after write might result in an
error on systems without pread() and pwrite().

Fortunately, at the moment there are no widely used systems without these
syscalls.

At least such behavior was observed with CephFS, see:
http://mailman.nginx.org/pipermail/nginx/2015-July/048188.html.

Iterating through all connections takes a lot of CPU time, especially
with large number of worker connections configured.  As a result
nginx processes used to consume CPU time during graceful shutdown.
To mitigate this we now only do a full scan for idle connections when
shutdown signal is received.

Transitions of connections to idle ones are now expected to be
avoided if the ngx_exiting flag is set.  The upstream keepalive module
was modified to follow this.

With this change it's no longer needed to pass -D_GNU_SOURCE manually,
and -D_FILE_OFFSET_BITS=64 is set to use 64-bit off_t.

Note that nginx currently fails to work properly with master process
enabled on GNU Hurd, as fcntl(F_SETOWN) returns EOPNOTSUPP for sockets
as of GNU Hurd 0.6.  Additionally, our strerror() preloading doesn't
work well with GNU Hurd, as it uses large numbers for most errors.