Extend signal-process and proced.el

Extend signal-process and proced.el

* doc/lispref/processes.texi (Signals to Processes):
Document changes in signal-process.

* etc/NEWS: Mention changes in proced.el and signal-process.

* lisp/proced.el (proced-signal-function): Declare it obsolete.
(proced-remote-directory): New user option.
(proced-mode): Adapt docstring.
(proced-send-signal, proced-renice): Handle interactive prefix argument.

* lisp/net/tramp.el (tramp-signal-process): New defun.  Add it to
`signal-process-functions'.

* src/process.c (Finternal_default_signal_process): New defun,
providing the hitherto existing implementation of Fsignal_process.
(Fsignal_process): Loop through Vsignal_process_functions.
(Vsignal_process_functions): New defvar.
(Qinternal_default_signal_process, Qsignal_process_functions):
Declare symbols.
(Sinternal_default_signal_process): Declare subroutine.

* test/lisp/net/tramp-tests.el (tramp-test31-signal-process): New test.