man: strip the comments in man page headers

They contained the date, which creates a problem with multiarch
packages, since the generated file would be different each time.
man,html: say 'systemd 198' in the header

This should help readers of the man or HTML pages know if the documentation
is out of date. An alternative to use a date generated from 'git log' was
considered, but since we try to keep user visible documentation up to date,
showing the project version should be enough.