Some precisements in Tramp's connection type handling

Some precisements in Tramp's connection type handling

* doc/misc/tramp.texi (Remote processes): Precise connection type
handling.

* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process):
* lisp/net/tramp.el (tramp-handle-make-process):
Fix :connection-type handling.
(tramp-action-show-and-confirm-message): Pacify byte compiler.

* lisp/net/tramp-compat.el (tramp-compat-ignore-error): New defmacro.

* test/lisp/net/tramp-tests.el (tramp-test29-start-file-process)
(tramp-test30-make-process): Extend tests.