Rewrite libhesiod0.postinst to remember debconf prompts

The changes allow libhesiod0 to actually use the values from the
debconf prompts by writing them out to hesiod.conf.  Unfortunately,
this currently has the side-effect of clobbering anything
config-package-dev does to /etc/hesiod.conf by entirely rewriting the
file.  This is suboptimal, but #debian-devel's reaction was very
confused, so perhaps there isn't a better answer.