Run ssh-agent in foreground mode.

Background:

In my previous commit, I changed the way that ssh-agent was
spawned. However, it was only working when ssh-ident was running
in debug mode. This is because ssh-agent needs to run in
foreground work in general to work.

In this change:

- ssh-agent is run with the -D flag to force being run in
  foreground mode. This still makes it run as a seperate
  process. It just fixes compatibility with ssh-add -c.

Author: s1kx

ssh-ident

@@ -778,7 +778,7 @@ class AgentManager(object):
 
     # Redirect shell instructions from stdout to agent-file and debug output to
     # STDOUT if in DEBUG mode.
-    command_flags = ""
+    command_flags = "-D"
     log_pipe = None
     if ShouldPrint(config, LOG_DEBUG):
         command_flags = "-d"