cmd/gotree: Do not count root directory
cmd/gotree: Support for -a flag
cmd/gotree: Support for -d flag
cmd/gotree: Don't print $PWD
fs/memfs: Refactor tree parsing out to TreeBuilder struct

Breaks public interface, the following gofmt commands takes care of 4/5
rewrites one must perform to fix ones code:

	gofmt -w -r 'TabTree -> UnmarshalTab' .
	gofmt -w -r 'Unix.Tree -> Unix.Decode' .
	gofmt -w -r 'UnixTree -> UnmarshalUnix' .
	gofmt -w -r 'ErrCustomTree -> ErrTreeBuilder' .

The remaining one is to change CustomTree to TreeBuilder, like in the
following example:

	- var ct CustomTree = func...
	+ var tb TreeBuilder = TreeBuilder{DecodeLine: func...