prefork: Ignore SIGINT in child. This fixes race-condition in signals handling

when httpd is runnning on foreground and user hits ctrl+c. In this case, SIGINT
is sent to all children followed by SIGTERM from the main process, which
interrupts the SIGINT handler and leads to inconsistency (process freezes
or crashes).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1618555 13f79535-47bb-0310-9956-ffa450edef68