closingd: add lightning_simpleclosed for option_simple_close

New subdaemon implementing the BOLT2 simple close protocol, replacing `lightning_closingd` when `option_simple_close` is negotiated:
- Each peer independently sends `closing_complete` with their fee proposal;
- The other side signs it and sends `closing_sig`;
- Both sides broadcast two conflicting closing transactions and whichever confirms first wins.

Key protocol details:
- Closer pays the fee and closee receives their exact channel balance;
- TLV variants selected per BOLT2: `closer_output_only`, `closer_and_closee_outputs`, `closee_output_only`;
- Sequence 0xFFFFFFFD enables RBF via re-sending `closing_complete`;
- Script mismatch on `closee_scriptpubkey` warns and fails to reconnect;

common/shutdown_scriptpubkey.h/c: removed `static` from `is_valid_op_return` so it can be used in `simpleclosed.c`

Author: nGoline

closingd/Makefile

@@ -8,15 +8,26 @@ CLOSINGD_SRC := closingd/closingd.c	\
 CLOSINGD_OBJS := $(CLOSINGD_SRC:.c=.o)
 $(CLOSINGD_OBJS): $(CLOSINGD_HEADERS)
 
+# Simple close daemon
+SIMPLECLOSED_HEADERS := closingd/simpleclosed_wiregen.h
+
+SIMPLECLOSED_SRC := closingd/simpleclosed.c	\
+	$(SIMPLECLOSED_HEADERS:.h=.c)
+
+SIMPLECLOSED_OBJS := $(SIMPLECLOSED_SRC:.c=.o)
+$(SIMPLECLOSED_OBJS): $(SIMPLECLOSED_HEADERS)
+
 # Make sure these depend on everything.
-ALL_C_SOURCES += $(CLOSINGD_SRC)
-ALL_C_HEADERS += $(CLOSINGD_HEADERS)
-ALL_PROGRAMS += lightningd/lightning_closingd
+ALL_C_SOURCES += $(CLOSINGD_SRC) $(SIMPLECLOSED_SRC)
+ALL_C_HEADERS += $(CLOSINGD_HEADERS) $(SIMPLECLOSED_HEADERS)
+ALL_PROGRAMS += lightningd/lightning_closingd lightningd/lightning_simpleclosed
 
 # Here's what lightningd depends on
-LIGHTNINGD_CONTROL_HEADERS += closingd/closingd_wiregen.h
-LIGHTNINGD_CONTROL_OBJS += closingd/closingd_wiregen.o
+LIGHTNINGD_CONTROL_HEADERS += closingd/closingd_wiregen.h closingd/simpleclosed_wiregen.h
+LIGHTNINGD_CONTROL_OBJS += closingd/closingd_wiregen.o closingd/simpleclosed_wiregen.o
 
 lightningd/lightning_closingd: $(CLOSINGD_OBJS) $(HSMD_CLIENT_OBJS) libcommon.a
 
+lightningd/lightning_simpleclosed: $(SIMPLECLOSED_OBJS) $(HSMD_CLIENT_OBJS) libcommon.a
+
 -include closingd/test/Makefile