New CHAP implementation, rewritten from scratch to avoid the code

New CHAP implementation, rewritten from scratch to avoid the code
copyrighted by the uncontactable Gregory Christy.  The new code is
much cleaner and splits out all the digest-specific code to separate
files.  Thus the CHAP-MD5 stuff is now in chap-md5.c and all the
CHAP-MS and CHAP-MSv2 stuff has moved into chap_ms.c, instead of
having half of it in chap.c.

There are a few minor differences in this implementation; we don't
retransmit responses, but instead just wait for a new challenge.
The success/failure messages are more boring as well.  In fact the
digest code now sets the success/failure message.

CHAP_DIGEST_MD5 has been renamed to CHAP_MD5 for consistency.
There is a new function random_bytes() in magic.c, which generates
a string of random bytes.