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.

Valgrind complains if we pass uninitialized memory to a syscall:

==36492== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==36492==    at 0x6B5E6A: sendmsg (in /usr/lib/system/libsystem_kernel.dylib)
==36492==    by 0x10004288E: ngx_signal_worker_processes (ngx_process_cycle.c:527)
==36492==    by 0x1000417A7: ngx_master_process_cycle (ngx_process_cycle.c:203)
==36492==    by 0x100001F10: main (nginx.c:410)
==36492==  Address 0x7fff5fbff71c is on thread 1's stack

Even initialization of all members of the structure passed isn't enough, as
there is padding which still remains uninitialized and results in Valgrind
complaint.  Note there is no real problem here as data from uninitialized
memory isn't used.

Valgrind intercepts SIGUSR2 in some cases, and nginx might not be able to
start due to sigaction() failure.  If compiled with NGX_VALGRIND defined,
we now ignore the failure of sigaction().

This fixes connection hang with websockets proxy, and likely some other
places as well.