[core] set signal handlers before forking child processes in modules/plugins_call_set_defaults (fixes #2502)

From: Stefan Bühler <stbuehler@web.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2901 152afb58-edef-0310-8abb-c4023f1b3aa9