Implement alternative for Tramp's signal return string

Implement alternative for Tramp's signal return string

* lisp/net/tramp-adb.el (process-file-return-signal-string): Declare.
(tramp-adb-get-signal-strings): New defun.
(tramp-adb-handle-process-file): Use it.

* lisp/net/tramp-sh.el (process-file-return-signal-string): Declare.
(tramp-sh-get-signal-strings): New defun.
(tramp-sh-handle-process-file): Use it.

* lisp/net/tramp.el (tramp-get-signal-strings): Remove function.

* test/lisp/net/tramp-tests.el (tramp-test28-process-file):
Accept alternative signal return string.