And now, to answer my own question....

William Allen Simpson wrote:
> 
> After reboot, the domain part of the hostname is lost.  This is a
> sysinst bug.
> 
> Perhaps the answer is to change the hostname= to use the fully
> qualified name?
> 
Yes, that's the answer.  In fact, /etc/default/rc.conf clearly says:

# Basic network configuration
#

# Fully Qualified Internet Domain Name (a.k.a. hostname, e.g. foo.baz.edu).
# If blank, use /etc/myname.
#
hostname=""

# If there's only one way out of your network, set this to the hostname
# or the IP address of the router that will get your packets off the LAN.
# If blank, use /etc/mygate.
#
defaultroute=""

# The NIS domain name (formerly known as Yellow Pages); not in any way
# related to Internet domain names.
# If blank, use /etc/defaultdomain.
#
domainname=""

-- 
William Allen Simpson
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