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.

The cycle->new_log->log_level should only be initialized by ngx_init_cycle()
if no error logs were found in the configuration.  This move allows to get rid
of extra initialization in ngx_error_log().

If "stderr" was specified in one of the "error_log" directives,
stderr is not redirected to the first error_log on startup,
configuration reload, and reopening log files.

On win32 stderr was not redirected into a file specified by "error_log"
while reopening files.  Fix is to use platform-independent functions to
work with stderr, as already used by ngx_init_cycle() and main() since
rev. d8316f307b6a.

It is now a syntax error if tokens passed to a custom configuration
handler are terminated by "{".

The following incorrect configuration is now properly rejected:

  map $v $v2 {
      a b {
      c d {
      e f {
  }

No functional changes.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>