mod_slotmem_*: Add in new fgrab() function which forces a grab and

slot allocation on a specified slot. Allow for clearing of inuse
array.

trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1386880
             http://svn.apache.org/viewvc?view=revision&revision=1387389
             http://svn.apache.org/viewvc?view=revision&revision=1388445
             http://svn.apache.org/viewvc?view=revision&revision=1394079
Submitted by: jim
Reviewed by: rjung, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1422580 13f79535-47bb-0310-9956-ffa450edef68