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

@@ -62,6 +62,8 @@ The origin node:
   * If it requires a feature above, MUST set the corresponding even
     feature bit in all feature fields indicated by the Context column,
     unless indicated that it must set the odd feature bit instead.
+  * If a feature is specified both as an optional feature and as a
+  required feature in the header, the more restrictive option MUST be chosen.
   * MUST NOT set feature bits it does not support.
   * MUST NOT set feature bits in fields not specified by the table above.
   * MUST set all transitive feature dependencies.