Fixed AIO error handling on FreeBSD.

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.

Fixed sched_setaffinity(2) to correctly pass size.

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.

Microoptimization of sendfile(2) usage under FreeBSD.

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.

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

Configure/build changes and fixes:

*) Revamped "configure --help" text.

*) FreeBSD 10-current has recently gotten POSIX_FADV_* macros.
   A fix for the broken build applied.

*) AIX 7 defines sys_nerr in errno.h, therefore <errno.h> included
   in the sys_nerr test.

   When sys_nerr and _sys_nerr are missed on a particular platform
   and our euristic for a maximum errno detection applied always
   print the maximum errno number we reached instead of printing void.

   This makes possible to build nginx on AIX 7.1.

*) Made it possible to build/install from the SVN checkout.

Fixed range checking for the "somaxconn" sysctl.  Fixed port range checking.

Fixed unix ngx_write_chain_to_file() to return total bytes written.

Previously result of last iteration's writev() was returned.  This was
unnoticed as return value was only used if chain contained only one or
two buffers.

Various fixes related to error messages:

*) Removed old warning that suggested to use "server_name_in_redirect off"
   (now the default) in place of no longer supported "server_name *".

*) Fixing directive name in error message if types hash size is not enough.

*) Replaced "can not" with "cannot" and "could not" in a bunch of places.
   Fixed nearby grammar errors.

*) Overhauled error messages.

SIGWINCH/NOACCEPT signal is disabled now in non-daemon mode.
Non-daemon mode is currently used by supervisord, daemontools and so on
or during debugging. The NOACCEPT signal is only used for online upgrade
which is not supported when nginx is run under supervisord, etc.,
so this change should not break existant setups.