Adding ability to set backtrace depth for debug_backtrace()

To determine a correct filename and line number, where user actually requested data from.

Also, DEBUG_BACKTRACE_IGNORE_ARGS stays there, as it saves memory. Can be deleted for PHP versions older than 5.3.6

Signed-off-by: Peter Legierski <peter@legierski.net>
Merge branch 'refs/heads/hotfix-backtrace-depth'