Add DEVELOPER flag, set by default.

This is a bit messier than I'd like, but we want to clearly remove all
dev code (not just have it uncalled), so we remove fields and functions
altogether rather than stub them out.  This means we put #ifdefs in callers
in some places, but at least it's explicit.

We still run tests, but only a subset, and we run with NO_VALGRIND under
Travis to avoid increasing test times too much.

See-also: #176
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/dev_disconnect.c

@@ -11,6 +11,7 @@
 #include <unistd.h>
 #include <wire/gen_peer_wire.h>
 
+#if DEVELOPER
 /* We move the fd IFF we do a disconnect. */
 static int dev_disconnect_fd = -1;
 static char dev_disconnect_line[200];
@@ -132,3 +133,4 @@ void dev_blackhole_fd(int fd)
 	dup2(fds[1], fd);
 	close(fds[1]);
 }
+#endif