Improve man page layout.

Improve man page layout.

This changes the output of --help, --detailed-help and --version
so that the man pages generated from this output by help2man look
much nicer.

Specifically, name and purposes are printed in a single line and
the version text now contains also build data, build system and the
compiler version.

Also the codename is no longer printed in version_single_line(),
since this caused the top and bottom labels of the man pages to
look quite ugly.