fmt: split doPrint into two specialized functions

fmt: split doPrint into two specialized functions

Remove format flag reset from doPrint. Flags will not be set in
doPrint and printArg will not return with flags modified.

Remove the extra arguments addspace and addnewline and split up
doPrint into two simpler and specialized functions.

Change-Id: Ib884d027abfbb31c6f01b008f51d6d76fc0c1a17
Reviewed-on: https://go-review.googlesource.com/21181
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>