geom: Allow "load" command for already loaded modules.

I see more user-friendly to do nothing if the module is already
loaded, rather than returning quite confusing error message.

As side effect it allows to avoid std_list_available() call, using
quite expensive on large systems geom_gettree().

MFC after: 1 month

(cherry picked from commit 5678114cd8b310bd6f0a5699f036fc5b18addd65)