Allow for custom manpage section

[anpep]

A way for providing a custom manpage section could prove very useful so that we don't depend on hacks on the application side to fix this manually.

go-flags/man.go

Line 197 in 105d09a

fmt.Fprintf(wr, ".TH %s 1 \"%s\"\n", manQuote(p.Name), t.Format("2 January 2006"))

Example hack on canonical/pebble:
https://github.com/canonical/pebble/blob/8e96c74a21cc104ac722bf4160d7db6e11ddeb60/internals/cli/cmd_help.go#L93-L120

[thp-canonical]

This also affects snap help --man:
https://github.com/snapcore/snapd/blob/d3d978da7b2089186cb8b9e492198a3090d65dc2/cmd/snap/cmd_help.go#L103-L130

One option (if we can't store/retrieve the section information in some other place?) could be to add a new function to man.go:

func (p *Parser) WriteManPageForSection(wr io.Writer, section string) {
  ...
}

And update the existing function (for existing user code) to just:

func (p *Parser) WriteManPage(wr io.Writer) {
    WriteManPageForSection(wr, '1')
}

Pebble and snap could then call WriteManPageForSection(out, '8').

Together with the new groff in 16.04 (see LP#1814767 -- 14.04 will go out of support next year?), this would allow the manfixer in pebble and snap to be removed.