src/libadacl.c: rewrite acl_set_entry() as acl_update_entry().

It turns out we only need to update existing entries in our ACLs, to
mask the execute permissions. Since we never need to create new
entries, the name "acl_set_entry" was not quite right. The
new-entry-creation code has been removed from the bottom half of the
function, and it has been renamed to "acl_update_entry".