Detect network mounts and skip slow operations while listing them.

On NFS/SMB/SSHFS, KIO no longers counts items in directories (e.g.
for dolphin in detailed view mode), nor does it look at subdir/.directory
files for custom icons or comments.

This commit will improve speed greatly in kdelibs-4.7.4 and later.
There's just one issue remaining, when navigating away from a directory
with many items, but that's a TODO for afiestas -- if he agrees :)