Merge r1821504, r1821505 from trunk:

mpm_{event,worker}: Mask signals for threads created by modules in child init.

PR 62009, so that they don't receive (implicitely) the ones meant for the MPM.

Inspired by: Armin Abfalterer <a.abfalterer gmail.com>


Follow up to r1821504: same comment in event than in worker.


Proposed by: ylavic
Reviewed by: ylavic, icing, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1821517 13f79535-47bb-0310-9956-ffa450edef68