Default dns_canonicalize_hostname to "fallback"

Default dns_canonicalize_hostname to "fallback"

This change should mitigate some of the pain caused by the rdns=true
default (generally associated with unwanted PTR records that cannot
easily be changed), with a minimum of fallout.

Update the documentation and tests accordingly.  In test environments,
disable qualify_shortname and use the uncanonicalized system hostname
(lowercased) to match the initial sn2princ result.

ticket: 8911 (new)