tabs for indentation even if -spaces is set.

Changes to gofmt:
- added -tabindent flag
- don't recompute parser and printer mode repeatedly

Changes to go/printer:
- provide new printing mode TabIndent

Changes to tabwriter:
- implement new mode TabIndent to use tabs independent
  of the actual padding character for leading empty columns
- distinguish between minimal cell width and tab width
  (tabwidth is only used if the output contains tabs,
  minwidth and padding are always considered)
- fixed and added more comments
- some additional factoring

By default, -tabindent is disabled and the default gofmt
behavior is unchanged. By setting -spaces and -tabindent,
gofmt will use tabs for indentation but do any other
alignment with spaces. This permits a user to change the
visible indentation by simply changing the editor's tab
width and the code will remain properly aligned without
the need to rerun gofmt.

R=rsc
https://golang.org/cl/163068