ppp: Permit CHAP challenges up to 255 bytes

[jhb]

Summary

RFC 1994 does not place any limit on the length of the value field in
challenge messages except that the length is a single octet which
bounds the maximum length to 255.

NB: I'm not sure why the local[] and peer[] arrays contain room for an
authentication name (AUTHLEN) in addition to a challenge value/response,
but I've just left that in place.

PR: 271955
Reported by: Robert Morris <[e-mail]>

[jhb]

usr.sbin/ppp/chap.c

241

The cast is required so that the single char is promoted to an unsigned int instead of a signed int when passed as the length.

[des]

It's safe to drop the extra AUTHLEN from challenge.local and challenge.peer

[jhb]

One more cast