Avoid cyclic dependency by moving ap_set_etag() from module http to core.

Avoid cyclic dependency by moving ap_set_etag() from module http to core.

This function, along with ap_make_etag(), is used by the default_handler in
core.c, and in several modules other than builtin mod_http, breaking static
linking and httpdunit tests build.

The move is done by "svn move modules/http/http_etag.c server/util_etag.c".
MMN major bumped, not backportable (as is) to 2.4.x.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1833083 13f79535-47bb-0310-9956-ffa450edef68