fix negative time in "Cache-Control" if "expires modified" sets time in the past

perl exit fixes:

*) use global perl variable in perl_destruct()/perl_free()
   for non-mulitiplicity perl
*) add comment from r2716 commit message

gzip headers related fixes:

*) remove "Content-Encoding: gzip" in 304 response
   sent by ngx_http_gzip_static_module
*) refactor gzip_vary handling
*) test r->header_only last, since it's not actually
   frequent here: 304 and HEAD responses are not set it before the filter

Fix a bug introduced in r2032: After a child process has read a terminate
message from a channel, the process tries to read the channel again.
The kernel (at least FreeBSD) may preempt the process and sends a SIGIO
signal to a master process. The master process sends a new terminate message,
the kernel switches again to the the child process, and the child process
reads the messages instead of an EAGAIN error. And this may repeat over
and over. Being that the child process can not exit the cycle and test
the termination flag set by the message handler.

The fix disallow the master process to send a new terminate message on
SIGIO signal reception. It may send the message only on SIGALARM signal.

cache related fixes:

*) do not pass buf with empty cached response,
   this fixes "zero size buf in output" alert
*) hide cacheable Set-Cookie and P3P FastCGI response headers
*) test comma separator in "Cache-Control"
*) a cache manager thread handle was overwritten by a cache loader thread
   handle, this caused an exit delay, the bug had been introduced in r3248
*) fix handling cached HTTP/0.9 response
*) log proxied HTTP/0.9 responses status as "009"
*) fix the "If-None-Match" header name
*) fix a cached zero-length body case

use setproctitle("%s", title)

fix segfault if $limit_rate was logged
fix segfault in SSL if limit_rate is used

fix "if (!-x ...)"

      r3202, r3203, r3383, r3388, r3417, r3418:

Win32 (mostly) and some Unix file related fixes:

*) preserve errno while ngx_free()
*) win32 ngx_file_info() utf8 support
*) delete Win95 code
*) log file name for read/write errors
*) test incomplete WriteFile()
*) handle short pwrite() to log an error cause: ENOSPC, EDQUOT, or EFBIG
*) uniform ngx_directio_on/off() interface with other file functions
*) do not create Win32 drive letter in ngx_create_full_path()
*) ignore EACCES errors for top level directories in ngx_create_full_path()
*) fix Win32 error messages when an temporary file replaces an existent file:
   *) do not rename an already renamed file
   *) now ngx_win32_rename_file() returns error code
   *) do not log failure inside ngx_win32_rename_file()
*) fix Win32 error message when an temporary file replaces an existent file:
   return at once if ngx_win32_rename_file() was not failed
   and do not try to delete already the renamed temporary file
*) skip URI trailing spaces under Win32
*) disable Win32 short file names