homework: allow specifying explicit additional mount options when using CIFS backend

This is useful since certain shares can only be mounted with additional
mount flags. For example the SMB share in modern AVM Fritz!Boxes
requires "noserverino" to be set to work from Linux.

src/shared/user-record.h

@@ -304,6 +304,7 @@ typedef struct UserRecord {
         char *cifs_domain;
         char *cifs_user_name;
         char *cifs_service;
+        char *cifs_extra_mount_options;
 
         char *image_path;
         char *image_path_auto; /* when none is configured explicitly, this is where we place the implicit image */