common: allow configvars to be marked dynamic.

To test, we do min-capacity-sat which is simple.  We also update the
listconfigs man page which contained some obsolete information.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/configvar.h

@@ -58,6 +58,8 @@ struct configvar {
 #define OPT_SHOWMSATS (1 << (OPT_USER_START+4))
 /* listconfigs should treat as a literal boolean `true` or `false` */
 #define OPT_SHOWBOOL (1 << (OPT_USER_START+5))
+/* Can be changed at runtime */
+#define OPT_DYNAMIC (1 << (OPT_USER_START+6))
 
 /* Use this instead of opt_register_*_arg if you want OPT_* from above */
 #define clnopt_witharg(names, type, cb, show, arg, desc)		\