Bump default MMAP_THRESHOLD to 1 from 0 because some systems don't allow

mmap of size 0.

PR:		1491
Reviewed by:	Jim Jagielski, Ken Coar


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79660 13f79535-47bb-0310-9956-ffa450edef68