server: explicitly call "shutdown" when catch CTRL-C or a signal

server: explicitly call "shutdown" when catch CTRL-C or a signal

Every TCL command can be renamed (or deleted) and then replaced by
a TCL proc that has the same name of the original TCL command.
This can be used either to completely replace an existing command
or to wrap the original command to extend its functionality.
This applies also to the OpenOCD command "shutdown" and can be
useful, for example, to set back some default value to the target
before quitting OpenOCD.
E.g. (TCL code):
	rename shutdown original_shutdown
	proc shutdown {} {
		puts "This is my implementation of shutdown"
		# my own stuff before exit OpenOCD
		original_shutdown
	}

Unfortunately, sending a signal (or pressing CTRL-C) to terminate
OpenOCD doesn't trigger calling the original "shutdown" command
nor its (eventual) replacement.

Detect if the main loop is terminated by an external signal and
in such case execute explicitly the command "shutdown".
Replace with enum the magic numbers assumed by "shutdown_openocd".

Please notice that it's possible to write a custom "shutdown" TCL
proc that does not call the original "shutdown" command. This is
useful, for example, to prevent the user to quit OpenOCD by typing
"shutdown" in the telnet session.
Such case will not prevent OpenOCD to terminate when receiving a
signal; OpenOCD will quit after executing the custom "shutdown"
command.

Change-Id: I86b8f9eab8dbd7a28dad58b8cafd97caa7a82f43
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4551
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>