server: support binding to arbitrary interfaces

Some installations of OpenOCD are used in restricted environments that
do not permit binding to public interfaces.

This patch does not affect the default behavior to listen on all
interfaces, however it does give the option to restrict services by way
of the bindto command.

Change-Id: Id51bd64b376a8c62dd47b08b4d834872925e6af2
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/3534
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

doc/openocd.texi

@@ -6731,6 +6731,11 @@ the initial log output channel is stderr.
 Add @var{directory} to the file/script search path.
 @end deffn
 
+@deffn Command bindto [name]
+Specify address by name on which to listen for incoming TCP/IP connections.
+By default, OpenOCD will listen on all available interfaces.
+@end deffn
+
 @anchor{targetstatehandling}
 @section Target State handling
 @cindex reset