Interactive socket selection for `rdbg -A`

[st0012]

selectable.sockets.mov

Code change:

         else
-          $stderr.puts "Please select a debug session:"
-          files.each{|f|
-            $stderr.puts "  #{File.basename(f)}"
-          }
-          exit
+          require "cli/ui"
+          CLI::UI::Prompt.ask('Please select a debug session:') do |handler|
+            files.each do |file|
+              handler.option(file) { |selection| @s = Socket.unix(selection) }
+            end
+          end
         end