Add missing store v1 to v2 migration index

This significantly speeds up migrations from v1 to v2.  In my local test,
I was trying to upgrade a database with ~500 test runs and the original
process would not finish in a reasonable amount of time.
Plumb through support for required_disk_space

This new property will allow a test case to pre-specify the amount
of available disk space required for the test to run.  ATF-based
test cases provide this via a require.diskspace property.

This is still unused.
Pass the work directory to check_reqs()

Extend the check_reqs() function to receive the directory the test case
is being run in.  Will later be used to implement the required_disk_space
support.
Add the mount_tmpfs function

Add a function to mount a tmpfs on a given directory.  This is strictly
for tests at this point, but I'm putting the code under the regular fs
module for simplicity (easier to share code among the tests that need it
and easier to test it).

As part of this, add a new unsupported_operation_error error type to the
fs module and a new UTILS_UNCONST macro.
Add the unmount function

Add a function to unmount a file system using either unmount(2) or
umount(8) depending on the underlying operating system.  Just borrow
the already-tested code from the testers module.
Add the free_disk_space function

This new function queries the available free space, in bytes, of a given
file system.
Enforce the required_disk_space requirement

Recognize the new required_disk_space metadata property (provided
as require.diskspace from ATF test programs) and enforce it in the
requirements check.
Merge branch 'add-required-disk-space'

Add support to specify the amount of physical disk space required
by a test case.  This is in the form of a new "required_disk_space"
metadata property, which can also be provided by ATF test cases as
"require.diskspace".
Remove testers documentation

Having our "end-user" documentation point at the manual pages of the
testers only causes confusion.  (It confuses me, so it must also confuse
users.)  Because the tester binaries are internal at this point and I
have plans to kill them, just remove their documentation for simplicity.

While doing this, assimilate any useful remains into our other
documentation.  In particular, the only details that are worth keeping
are the fact that we now have support for TAP test programs and the list
of supported metadata properties.  These should all have gone into
kyuafile(5) in the first place.
Add a new kyua-test-isolation(7) page

This new page describes the environment in which each test case or test
program is run.  The contents come from the existing kyua-atf-interface(7)
page.
Remove the kyua-*-interface(7) pages

These pages are quite useless and only add confusion.  The definition of
each test interface belongs to the provider of such interface.

For example: ATF provides the atf(7) manual page (and supporting contents
such as atf-test-case(4) and atf-test-program(4)) and TAP is widely
documented on the Internet in various forms.  Kyua should just try to
support these upstream interfaces without re-explaining what they are.
Fix bad Xr entry
Add references to kyuafile(5)

While this manpage may have been discoverable on its own, it really
needs to be linked from many different places.  Add such links now.
Merge branch 'assimilate-testers-docs'

This removes all manual pages from the testers/ subdirectory by
assimilating their useful contents into other manpages, and also brings
in some miscellaneous documentation fixes.
Add an EXAMPLES section to kyuafile(5)

Move the sample Kyuafile from the beginning of the manual page to a new
EXAMPLES section.  While doing so, tweak the example and add a simpler
code sample.
Clarify include() restrictions in kyuafile(5)

Make it explicitly clear that include() in Kyuafiles cannot refer to
absolute paths and explain why.

Add a new example to describe how a user might connect disjoint test
suites into a single one so that Kyua can run them all together.
Merge branch 'issue-114'

Improve the kyuafile(5) manpage by adding more examples and clarifying
the behavior of include().

Fixes #114.
Deprecate per-test program test_suite overrides

Stop mentioning the ability to override the test_suite value on a test
program basis.  This is needless complexity that should never have been
added.

Do not actually remove support for this because, even though it is
probably not used anywhere in the wild, doing the change would require
a Kyuafile version bump and that is unwarranted for now.

Addresses part of #55.
Add an EXAMPLES section to kyua.conf(5)

Move the sample configuration files to a new EXAMPLES section and add
some more sample code.
Add ostream formatting for std::map