'pipex_mppe' and 'pipex_session' structures have uint16_t bit fields

'pipex_mppe' and 'pipex_session' structures have uint16_t bit fields
which represent flags. We mix unlocked access to immutable flags with
protected access to mutable ones. This could be not MP independent on
some architectures, so convert these fields to u_int `flags' variables.

ok bluhm@