mod_slotmem_shm: follow up to r1702450.

Make systems without unlink() semantic happy by destroying (detaching) the SHM
before removing the base file.
Windows is supposed to have the "delete on last handle closed" semantic but yet
fails here when this handle is owned by the same process (go figure!).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1703169 13f79535-47bb-0310-9956-ffa450edef68