Fix remote-host directory tracking for shells in `term' buffers

Fix remote-host directory tracking for shells in `term' buffers

* lisp/term.el (term-handle-ansi-terminal-messages): Use an explicit
tramp method when constructing the tramp path for a non-local host,
as this is now mandatory.  "-" is a pseudo-method for the user's
`tramp-default-method'.  (Bug#31355)

Specify the remote username explicitly in all cases, as
`tramp-default-user' and `tramp-default-user-alist' could cause the
previous logic to fail.

Minor related improvements to the commentary.