Fixed compile-time conditionals used to detect if X-Forwarded-For support
is needed.

Note: compatibility shims were added during merge to avoid possible
breakage of 3rd party modules.  At least cache purge module was broken
by the original commit, as it used to rely on NGX_HTTP_PROXY define.

HP-UX needs _HPUX_ALT_XOPEN_SOCKET_API to be defined to be able to
use various POSIX versions of networking functions.  Notably sendmsg()
resulted in "sendmsg() failed (9: Bad file number)" alerts without it.

See xopen_networking(7) for more details.

This fixes alignment problems observerd on ARMs, and likely also needed
for MIPSes.  Unless we know alignment is not required just assume we
need 16, which appears to be safe default for all architectures.

See here for details:
http://mailman.nginx.org/pipermail/nginx/2012-June/034139.html

*) Fixed spelling of "endianness", and called it "byte ordering" in the
   user visible part.

*) Fixed return type of ngx_strerror_init().

*) Fixed a harmless error in spelling of "Connection: close" when computing
   the response header length.

*) Style.

*) Added code to look up Google perftools in /opt/local/, for MacPorts.

*) Upstream: least_conn balancer module.

*) Upstream: weights and IPv6 support in ip_hash balancer.

*) Upstream keepalive: "single" parameter deprecated.

Fixed the ngx_regex.h header file compatibility with C++.  Fixed
building --with-cpp_test_module on some systems.

Broken by r4560.

Steps to reproduce:

./configure --with-cc="gcc46" --with-cc-opt="-Wall -Werror -O2"

systems with glibc versions prior to 2.7.  Fixed this by checking
the existence of "struct tcp_info" members during configuration.