all: format with Go 1.19

[charlievieth]

Update Go source files to use the '//go:build' lines introduced by Go 1.17 and the doc comment formatting introduced by Go 1.19. This formatting is not changed by Go 1.20.

Note: doc.go and the *userauth* files had to be manually changed in some places to fix indentation.

This does not change how godoc (at least with Go 1.19) renders the package documentation and is backwards compatible (i.e. earlier versions of gofmt will leave this code unchanged - tested with 1.17).