Write log output through StatusOutput so that the status implementation
can synchronize it with its own output.
Test: status_test.go
Change-Id: I917bdeeea4759a12b6b4aa6d6d86ee18a2771723
terminal.Writer is now just a wrapper around stdio.Stdout() without
any useful functionality. Replace it with stdio.Stdout() as an
io.Writer.
Test: ui/terminal/status_test.go
Change-Id: I5bc5476afdca950b505642f0135a3af9d37fbe24
Support for smart and dumb terminals are implemented in writer.go,
which makes dumb terminals much more complicated than necessary.
Move smart and dumb terminals into two separate implementations
of StatusOutput, with common code moved into a shared formatter
class.
Test: not yet
Change-Id: I59bbdae479f138b46cd0f03092720a3303e8f0fe