vote on exception hook backport.

review was fine, extra credit for cleaning up the duplication of signal
handling between the mpms.  Tested on RH9 w/prefork - no regression,
worker - no regression, enabled mod_backtrace here which worked as expected
once I set EnableExceptionHook On in httpd.conf.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@102782 13f79535-47bb-0310-9956-ffa450edef68