Permit UTF-8 names & passwords with CRAM-MD5 authentication

CRAM-MD5 (RFC 2195) permits 8-bit data but does not identify its encoding.
Since ASCII does not permit 8-bit data, this commit changes the encoding
to UTF-8.

There is an expired Internet-Draft that proposed that the RFC be changed
to explicitly require UTF-8 encoding of user names and shared secrets.
(But then there's also an expired draft proposing that CRAM-MD5 be retired
to historic status.)

Instead of CRAM-MD5, a better option for users is the SASL PLAIN mechanism
(within TLS) which explicitly permits UTF-8.