Simplify Tramp's find-executable

Simplify Tramp's find-executable

* lisp/net/tramp-cache.el (with-tramp-saved-connection-property):
Fix typo.

* lisp/net/tramp-compat.el: Add TODO.

* lisp/net/tramp-sh.el (tramp-find-executable): Simplify, using
"type -P ...".
(tramp-set-remote-path): Better handling of superlong $PATH.
(tramp-get-remote-path): Adapt/use connection properties.

* test/lisp/net/tramp-tests.el (tramp--test-enabled)
(tramp-test03-file-name-host-rules): Don't wrap
`tramp-cleanup-connection' with `ignore-errors'.

(cherry picked from commit ec34bccfee68a521e7a98ce8dce9325a146d095b)