bol09: Specify behavior when a node specifies both optional and requi…

…red features

While reviewing a patch on lnprototest, I encountered a scenario
where the BOLT 9 specification needed to provide clear guidance.

As a result, this commit adds specific requirements to
determine the appropriate behaviour when a node specifies
both optional and required features.

Additionally, if this situation appears to be an
implementation bug, it will be taken care of accordingly.

Reported-by: lnprototest
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

09-features.md

@@ -64,11 +64,16 @@ The origin node:
     unless indicated that it must set the odd feature bit instead.
   * MUST NOT set feature bits it does not support.
   * MUST NOT set feature bits in fields not specified by the table above.
+  * MUST NOT set both the optional and mandatory field.
   * MUST set all transitive feature dependencies.
 
 The origin node MUST support:
   * `var_onion_optin`
 
+The received node:
+  * if both the optional and the mandatory feature bits are set,
+  the feature should be treated as mandatory.
+
 The requirements for receiving specific bits are defined in the linked sections in the table above.
 The requirements for feature bits that are not defined
 above can be found in [BOLT #1: The `init` Message](01-messaging.md#the-init-message).