Add the doc/manbuild.sh script

This script wraps the logic to build a manual page from sources.  At the
moment, it is just a very complex wrapper over sed but additional logic
will be added.

This also adds tests to make sure that the manual pages we generate are
valid.  In particular, we unit-test this new script and the script itself
ensures that no patterns are left unreplaced in the generated files.
Add support for file inclusion to manbuild

Extend the doc/manbuild.sh script to recognize an "include" directive
so that we can build manpages from various source files.
Hide sed expressions in manbuild

Take the list of replacements as var=value pairs instead of sed arguments.
This hides the expressions and the '__' prefixes and suffixes to variables,
which in turn guarantees that our validate_file function is searching for
the right substrings.
Remove the kyua-test-filters(7) manpage

This manual page is auxiliary and very short, so it is distracting to keep
around.  Fold the contents into the pages that need a description of test
files, and do so using our shiny new "file inclusion" functionality in
manbuild.
Remove the kyua-build-root(7) manpage

Fold the contents of kyua-build-root(7) into all the pages that need a
description of build directories.  This makes the contents more clear
as they can refer to the calling command instead of being "generic" and
talking about kyua(1) as a whole.
Describe --results-file in each command

Move the "abstract" description of the --results-file flag supported by
various commands from the kyua-results-files(7) manpage to the manpage
of every command that supports the flag.
Remove the kyua-test-isolation(7) manpage

Fold the contents of kyua-test-isolation(7) into the commands that run
tests so that the information is readily available and more discoverable.
Remove the kyua-results-files(7) manpage

Fold the remaining contents of kyua-results-files(7) into all the places
where they belong.
Fix manbuild to preserve formatting

Make sure manbuild preserves trailing space and backslashes in parsed
files.
Drop stale contents from .gitignore
Merge branch 'manbuild'

This merge assimilates the contents of all auxiliary manual pages,
including kyua-build-root(7), kyua-results-files(7), kyua-test-filters(7)
and kyua-test-isolation(7), into the relevant command-specific manual
pages.  This is for easier discoverability of relevant information when
reading how specific Kyua commands work.

To do this, we now have a new "manbuild" script that is in charge of
preparing a manual page from a bunch of source files.  In particular,
this script performs variable replacement, handles file inclusion and
executes some sanity checks on the generated text.