libwebsockets

sspc: add translation helpers and comments about pss usage between client and proxy

Helpers remove casts and derefs.

Add additional pointer arithmetic in client_pss_to_sspc_h() helper to
remove dependency on handle_offset being the first thing in the userdata

Make the helper names explicit for different proxy and client pss handling,
so it should be clearer that client helpers belong in a client section and
vice versa.