MINOR: cli: add a new "show env" command

Using environment variables in configuration files can make troubleshooting
complicated because there's no easy way to verify that the variables are
correct. This patch introduces a new "show env" command which displays the
whole environment on the CLI, one variable per line.

The socket must at least have level operator to display the environment.
diff --git a/include/types/applet.h b/include/types/applet.h
index 2e1626a..562575f 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -133,6 +133,9 @@
 		struct {
 			struct proxy *backend;
 		} server_state;
+		struct {
+			char **var;
+		} env;
 	} ctx;					/* used by stats I/O handlers to dump the stats */
 };