Whitespace fixes.

- Add an ERRORS section rather than incompletely documenting the
possible errno values in the DESCRIPTION section.

- Use .Vt for variable types outside the SYNOPSIS section rather than .Ft
- Use .Fn for function names outside the NAME section rather than .Nm
- Mark NULL as a defined value (.Dv)
- New sentence new

- Use .Vt for variable types outside the SYNOPSIS section rather than .Ft
- Use .Fn for function names outside the NAME section rather than .Nm
- Mark NULL as a defined value (.Dv)
- New sentence new line
- Don't start sentences with an arugment name
- Use \- rather than a bare - for a minus sign
- Spelling: rightmost
- Stray whitespace
(Most changes from FreeBSD)

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Convert TNF licenses to new 2 clause variant

Add the remaining files from net that are going to be moved here and
the Makefile glue.