Move constant ALPHANUMERIC into function isValidRouterNickname().
Move constant ALPHANUMERIC into function isValidRouterNickname().

It isn't used anywhere else, and it would be silly to import it from
here, so there is no reason to have it at the module level.

mmaker/bridgedb,wfn/bridgedb,mmaker/bridgedb,pagea/bridgedb,pagea/bridgedb,wfn/bridgedb