Fix broken monky-get-tramp-root-dir

[mnick]

Currently, monky-get-tramp-root dir is broken. This patch applies the following changes

• convert output of tramp-dissect-file-name from list to vector before using aset
• replace 6th (not 3rd) entry in tramp-path with root dir
• correctly convert modified vector to tramp-make-tramp-file-name args

[Wilfred]

Looks correct, thank you :)

FWIW there's also a tramp-file-name-localname function (also works with setf), so you don't have to use aset with explicit indices.