The previous commit added a means to generating the completion scripts
and this one plugs that into the build system.
A new build option 'install_completions' has been introduced. Set to
'True' by default.
Completions for bash and fish use pkg-config for getting the preferred
install locations for the completions. If the packages are not
available, fallbacks are in-place.
The 'completion' subdir has been kept to work around the ideology of
Meson that does not allow creating/outputing files in subdirectories nor
using the output of custom_target() in install_data().
https://github.com/containers/toolbox/pull/840