cmd/dist: Make verbose messages print to stderr

cmd/dist: Make verbose messages print to stderr

Made the following changes:
 - Export errprintf() from all three OS-specific modules
 - Added errprintf() to a.h
 - Moved errprintf() in windows.c under xprintf(), since they are so similar
 - Replaced all instances of xprintf() with errprintf() where a vflag check is done
Fixes #3788.

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/6346056