charon-cmd: Add --agent option to authenticate using ssh-agent(1)

The socket path is read from the SSH_AUTH_SOCK environment variable.
So using this with sudo might require the -E command line (or an appropriate
sudoers config) to preserve the environment.

src/charon-cmd/cmd/cmd_options.c

@@ -35,6 +35,8 @@ cmd_option_t cmd_options[CMD_OPT_COUNT] = {
 	  "trusted certificate, for authentication or trust chain validation" },
 	{ CMD_OPT_RSA, "rsa", required_argument, "path",
 	  "RSA private key to use for authentication" },
+	{ CMD_OPT_AGENT, "agent", no_argument, "",
+	  "use SSH agent for authentication"},
 	{ CMD_OPT_LOCAL_TS, "local-ts", required_argument, "subnet",
 	  "additional traffic selector to propose for our side" },
 	{ CMD_OPT_REMOTE_TS, "remote-ts", required_argument, "subnet",