Remove kyua-atf-compat

This module has never been used as intended and its main consumer,
NetBSD, never found it suitable.  There are enough differences in
how Kyua behaves with respect to the old ATF tools that a good level
of drop-in compatibility is just too hard.

For that reason, the current plan of record for NetBSD is to just
enable both the old ATF tools and Kyua simultaneously, allowing
users to explicitly migrate form one to the other.

More importantly, the deprecated atf-run and atf-report tools were
removed from upstream on 2014-02-07.  Their code lives in the NetBSD
repository.
Merge kyua-cli and kyua-testers

Move all the sources of kyua-cli to the top-level of the tree to start
the process of offering a single package for Kyua.

To make this initial step simple, keep kyua-testers in a separate
'testers' subdirectory that we recurse into for configuration and build
purposes.
Merge testers project into the top-level directory

Fold all of the automake and autoconf files from the old kyua-testers
package (now in the testers subdirectory) into the build system of the
top-level directory.
Rename the package to 'kyua'

Stop calling ourselves kyua-cli.  Just use kyua as the package name.
Use relative includes in testers

The build system only points to the top-level source and build directories
by default when searching for include files.  Just use relative paths as
we do elsewhere in Kyua instead of trying to mess with the -I flags.  This
fixes "make distcheck".
Homogenize introduction to Kyua in various docs

Make sure all of the various READMEs and the kyua(1) manual page provide
a consistent introduction to Kyua (and one that does not refer to the
previous multi-module setup of the codebase).
Merge branch 'merge-components'

This change merges the 'kyua-cli' and 'kyua-testers' into a single
'kyua' package rooted at the top level of the source tree and drops
the useless 'kyua-atf-compat' package.
Avoid dropping privileges when not necessary

Do not try to drop privileges to unprivileged_user when we are already
running as an unprivileged user.  Doing so is not possible and thus causes
spurious test failures when the current user is not root and the current
user and unprivileged_user do not match.

Fixes #100.