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
failed #6787


passed #6788


Name Stage Failure
test-all Test
error: cannot lock ref 'refs/remotes/origin/master': Unable to create '/builds/emacs/emacs/.git/refs/remotes/origin/master.lock': File exists.

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fatal: failed to run reflog
ERROR: Job failed: exit code 1