message/flags: add flags_from_path

Borrow from mu-maildir, to which we cannot link.

lib/message/mu-flags.hh

@@ -278,7 +278,7 @@ flags_from_delta_expr(std::string_view expr, Flags flags,
  * @return either messages flags or Nothing in case of error.
  */
 constexpr Option<Flags>
-flags_from_expr(std::string_view            expr,
+flags_from_expr(std::string_view        expr,
 		Option<Flags> flags = Nothing)
 {
 	if (expr.empty())
@@ -291,6 +291,19 @@ flags_from_expr(std::string_view            expr,
 		return flags_from_absolute_expr(expr, true);
 }
 
+/**
+ * Get the Maildir flags from the full path of a mailfile. The flags are as
+ * specified in http://cr.yp.to/proto/maildir.html, plus MU_MSG_FLAG_NEW for new
+ * messages, ie the ones that live in new/. The flags are logically OR'ed. Note
+ * that the file does not have to exist; the flags are based on the path only.
+ *
+ * @param pathname of a mailfile; it does not have to refer to an existing
+ * file/message
+ *
+ * @return the message flags or Nothing
+ */
+Option<Flags> flags_from_path(const std::string& path);
+
 /**
  * Filter out flags which are not in the given category
  *