When configured with the "set_real_ip_from", it can set client's IP
address as visible in logs to the one obtained via the PROXY protocol.

The strerrordesc_np() function, introduced in glibc 2.32, provides an
async-signal-safe way to obtain error messages.  This makes it possible
to avoid copying error messages.

Previously, systems without sys_nerr (or _sys_nerr) were handled with an
assumption that errors start at 0 and continuous.  This is, however, not
something POSIX requires, and not true on some platforms.

Notably, on Linux, where sys_nerr is no longer available for newly linked
binaries starting with glibc 2.32, there are gaps in error list, which
used to stop us from properly detecting maximum errno.  Further, on
GNU/Hurd errors start at 0x40000001.

With this change, maximum errno detection is moved to the runtime code,
now able to ignore gaps, and also detects the first error if needed.
This fixes observed "Unknown error" messages as seen on Linux with
glibc 2.32 and on GNU/Hurd.

Reported by Thibault Nélis.

When installing or running from a non-root user it is sometimes required to
override default, compiled in error log path. There was no way to do this
without rebuilding the binary (ticket #147).

This patch introduced "-e" command line option which allows one to override
compiled in error log path.

Addon modules, both dynamic and static, can now use shared source files.
Shared sources result in only one make rule even if specified several
times in different modules.

Adds 'set' directive to the stream server context.

This makes it possible to avoid looping for a long time while working
with a fast enough peer when data are added to the socket buffer faster
than we are able to read and process them (ticket #1431).  This is
basically what we already do on FreeBSD with kqueue, where information
about the number of bytes in the socket buffer is returned by
the kevent() call.

With other event methods rev->available is now set to -1 when the socket
is ready for reading.  Later in ngx_recv() and ngx_recv_chain(), if
full buffer is received, real number of bytes in the socket buffer is
retrieved using ioctl(FIONREAD).  Reading more than this number of bytes
ensures that even with edge-triggered event methods the event will be
triggered again, so it is safe to stop processing of the socket and
switch to other connections.

Using ioctl(FIONREAD) only after reading a full buffer is an optimization.
With this approach we only call ioctl(FIONREAD) when there are at least
two recv()/readv() calls.