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

It was broken in 8e446a2daf48 when the NGX_SENDFILE_LIMIT constant was added
to ngx_linux_sendfile_chain.c having the same name as already defined one in
ngx_linux_config.h.

The newer is needed to overcome a bug in old Linux kernels by limiting the
number of bytes to send per sendfile() syscall.  The older is used with
sendfile() on ancient kernels that works with 32-bit offsets only.

One of these renamed to NGX_SENDFILE_MAXSIZE.

In ngx_*_sendfile_chain() when calculating pointer to a first
non-zero sized buf, use "in" as iterator.  This fixes processing
of zero sized buf(s) after EINTR.  Otherwise function can return
zero sized buf to caller, and later ngx_http_write_filter()
logs warning.

Based on a patch by Sepherosa Ziehau.

Currently this flag is needed for epoll and rtsig, and though these methods
usually present on different platforms than kqueue, nginx can be compiled to
support all of them.

On Linux x32 inclusion of sys/sysctl.h produces an error.  As sysctl() is
only used by rtsig event method code, which is legacy and not compiled
in by default on modern linuxes, the sys/sysctl.h file now only included
if rtsig support is enabled.

Based on patch by Serguei I. Ivantsov.

When several "error_log" directives are specified in the same configuration
block, logs are written to all files with a matching log level.

All logs are stored in the singly-linked list that is sorted by log level in
the descending order.

Specific debug levels (NGX_LOG_DEBUG_HTTP,EVENT, etc.) are not supported
if several "error_log" directives are specified.  In this case all logs
will use debug level that has largest absolute value.