Clean up size_t abuse, part 2. ap_malloc/calloc/realloc are explicitly

excluded from this cleanup as they must be signature identical to the
clib functions, and although the definition of size_t has been flakey,
the definition of those functions appears to be generally clean since
ANSI C.

Backports: r1228323

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1228324 13f79535-47bb-0310-9956-ffa450edef68