Improve connection type `pipe' for remote processes

Improve connection type `pipe' for remote processes

* doc/misc/tramp.texi (Remote processes): New subsection "Remote
process connection type".

* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
Use `tramp-process-connection-type' as default connection type.
Improve check for `:connection-type'.

* lisp/net/tramp-sh.el (tramp-sh-handle-make-process):
Use `tramp-process-connection-type' as default connection type.
Improve check for `:connection-type'.  Send "stty -icrnl" when
connection type is a pipe.

* lisp/net/tramp.el (tramp-process-connection-type): Allow all
possible values.
(tramp-handle-make-process): Use `tramp-process-connection-type'
as default connection type.  Improve check for `:connection-type'.

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