WebSocket: increase max unmask payload size to 256K and indicate that packet is truncated is going above the new limit

Bug: 10283
Change-Id: Iea5401d02bcf50a3427dce5f2b817c3b73a7dc11
Reviewed-on: https://code.wireshark.org/review/3098
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>