- add .Os, it is mandatory.
- don't redeclare .Nm when it's not needed.
- fix some warnings (checked with mandoc -Tlint).
- remove some leftover old stuff.
and mark it as finished in README.
One small rationale on the way the manpage is set up: Looking at
the coreutils manpage, it does not invite to be a quick reference
guide, whereas I wrote this manpage to be short and concise in regard
to the information the advanced user needs.
No one needs to explain what an octal number is. That's not part of
the scope of this manpage.
Also, nobody wants to read a block of text just to find out how
to build an octal mode string.
