Restore mod_slotmem_shm from 2.4.29.

Will restart from there to really fix PR 62308 (and PR 62044 still).

This effectively reverts:
- r1831394,
- r1830800,
- r1826970,
- r1826845,
- r1823572,
- r1823416,
- r1823415,
- r1823412,
- r1822511,
- r1822509.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1831868 13f79535-47bb-0310-9956-ffa450edef68