Recent Linux versions started to return EOPNOTSUPP to getsockopt() calls
on unix sockets, resulting in log pollution on binary upgrade.  Such errors
are silently ignored now.

This patch fixes incorrect handling of auto redirect in configurations
like:

    location /0  { }
    location /a- { }
    location /a/ { proxy_pass ... }

With previously used sorting, this resulted in the following locations
tree (as "-" is less than "/"):

        "/a-"
    "/0"    "/a/"

and a request to "/a" didn't match "/a/" with auto_redirect, as it
didn't traverse relevant tree node during lookup (it tested "/a-",
then "/0", and then falled back to null location).

To preserve locale use for non-ASCII characters on case-insensetive
systems, libc's tolower() used.

There is nothing to do more when recv() has returned 0,
so we should drop the flag.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.

Several false positive warnings silenced, notably W8012 "Comparing
signed and unsigned" (due to u_short values promoted to int), and
W8072 "Suspicious pointer arithmetic" (due to large type values added
to pointers).

With this patch, it's now again possible to compile nginx using bcc32,
with options we normally compile on win32 minus ipv6 and ssl.

Precompiled headers are disabled as they lead to internal compiler errors
with long configure lines.  Couple of false positive warnings silenced.
Various win32 typedefs are adjusted to work with Open Watcom C 1.9 headers.

With this patch, it's now again possible to compile nginx using owc386,
with options we normally compile on win32 minus ipv6 and ssl.

It was introduced in Linux 2.6.39, glibc 2.14 and allows to obtain
file descriptors without actually opening files.  Thus made it possible
to traverse path with openat() syscalls without the need to have read
permissions for path components.  It is effectively emulates O_SEARCH
which is missing on Linux.

O_PATH is used in combination with O_RDONLY.  The last one is ignored
if O_PATH is used, but it allows nginx to not fail when it was built on
modern system (i.e. glibc 2.14+) and run with a kernel older than 2.6.39.
Then O_PATH is unknown to the kernel and ignored, while O_RDONLY is used.

Sadly, fstat() is not working with O_PATH descriptors till Linux 3.6.
As a workaround we fallback to fstatat() with the AT_EMPTY_PATH flag
that was introduced at the same time as O_PATH.

This allows to build a directory listing whenever a loop exists in symbolic
link resolution of the path argument.