[0.85] factorize debug_backtrace, and allow to force it using Toolbox::backtrace()

git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@20803 35612d4c-9d10-0410-837c-bf9dd1be60d7
[0.85] clean

git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@20804 35612d4c-9d10-0410-837c-bf9dd1be60d7
[0.85] lighter backtrace output

git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@20805 35612d4c-9d10-0410-837c-bf9dd1be60d7