Judging from most current implementations (Smack, Conversations,
Gajim), most clients will use the domain name rather than the
fully qualified host name when constructing the DIGEST-MD5
digest-uri. Although this isn't conforming to RFC2831, interop
is of more value than 'doing the right thing'.

This commit makes SASL use the domain name when the mechanism
in play is DIGEST-MD5, and will use the host name for other
mechanisms.