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.
2 jobs for master in 40 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #6787
test-all

00:00:12

passed #6788
test-filenotify-gio

00:40:33

 
Name Stage Failure
failed
test-all Test
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