Proposed netcfg bug merge: DHCP/DNS data overrides preseeded values

[Matthew Palmer <mpalmer@debian.org>]

I'm grovelling over the BTS for netcfg a bit, and I've noticed that there
are a number of bugs that all cover the same ground: that preseeded values
for get_hostname/get_domain are ignored in favour of values taken from
DHCP/DNS (different bugs mention different sources).

Unless there are any objections, I'll merge the following bugs together into
one bug, 606636 (as it has the most discussion, as well as a patch that
seems like it solves the whole problem).

544513: dhcp returned hostname takes precedence                               
        has patch from 2009 that changes the default behaviour, making        
        preseeded value for get_hostname take precedence over DHCP
        responses. 
        ML discussion indicates this probably isn't acceptable.

343269: hostname/domain name preseeding quite broken
        No discussion since initial report
        
606636: Reverse DNS takes precedence over netcfg/get_hostname
        Lengthy discussion with patches for code and docs to add a new
	option which is the "highest priority" as a source of FQDN; this
        preserves existing behaviour and seems like the best possible
        compromise.
        
561573: Disable reverse-resolve via preseed
        Same problem, no discussion

- Matt