Add minimal DNS helper application

Add a minimal DNS resolver (built for both i386 and amd64) so that we
can stop using the dns-cgi hack.  This minimal DNS resolver uses
libares (c-ares).  It also is capable of validating an IPv4 address,
and returning its output in a format suitable for passing to a Bourne
shell "eval".

Put the necessary targets in the Makefile to build this.

Declare the "host" target as PHONY, and while we're at it, declare
other .PHONY targets that should have already been there ("clean" and
"install")
Don't hardcode IP/URL in debathena-loader.sh

Tweak debathena-loader.sh to get the URI of the preseed file and
transform it to the URL of the tarball, rather than hardcoding the IP
in yet another place.  This facilitates testing from other servers or
paths.
Dynamically generate debathena.preseed

Remove debathena.preseed from version control, and dynamically
generate it in the Makefile.  This allows for easy deployment to other
directories or servers for testing.  Declare the target as PHONY to
ensure it runs each time.  Also, remove the file in the "clean"
target.
Add kexec and install it in stage1

Rather than downloading an arbitrary kexec binary via wget, put it in
the installer here, and keep it under version control.

Add necessary phony Makefile targets to install this.
Rewrite stage1 installer to use debconf

Add a set of templates and rewrite the stage 1 installer to use
debconf, for a more pleasing UI, rather than relying on a
poorly-initialized VT5.  The logic is changed a bit, and we ask the
metapackage question here, as well as making it a bit easier to change
advanced options.
Convert debathena-loader.sh to debconf

Avoid switching to VT5 just to display an error message,
instead write out our own template file and use debconf.
Makefile: Tweaks to install target; variable names

- Switch from "d" to "DESTDIR" to better align with convention.
- Rename "INSTALLER" to "SUITE", which makes more sense.
- Add a target for DESTDIR which errors out if it doesn't exist.
- Modify the install target to depend on the DESTDIR target
- Cause the build to fail if DESTDIR isn't writable
- Add "-f" to "rm debathena.preseed" in clean
- Standardize on parens for variable expansion
- Simplify the arch-specific "host" target.
- Add LIBEXECDIR to define where in the tarball binaries go,
  and add a target for it (and roll the kexec/host targets
  into it)
- Drop some unused phony targets