Modify algorithm selection of rocks tree and add --local.

Drop old behavior in which the tree list was scanned to find the
first writable one (which meant users ended up installing rocks
locally). Now it always tries the last in the list (which is by
default the global one) and warns users when they have no write
permissions. (It picks the _last_ one so that the old behavior
when running sudo remains the same). Also add --local as a shorthand
to specify the local tree, as suggested by Steve Donovan.

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