'private/' subdir of BASEDIR should be created during 'tahoe create-alias' and friends

[warner]

user 'munderwo' on the IRC channel noticed an exception when they manually deleted their ~/.tahoe directory, then re-created it, then ran tahoe create-alias. It complained that the ~/.tahoe/private directory did not exist, which trying to write aliases.tmp.

I suspect that we create this directory during tahoe create-client, but then assume that it exists later. Instead, if tahoe create-alias notices that it is missing, it should just re-mkdir it.