os/user : use username as fullname if all else fails (on windows)

Fixes #4113.

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/6545054

Author: shivakumargn

src/pkg/os/user/lookup_windows.go

@@ -27,7 +27,9 @@ func lookupFullName(domain, username, domainAndUser string) (string, error) {
 		var p *byte
 		e = syscall.NetUserGetInfo(d, u, 10, &p)
 		if e != nil {
-			return "", e
+			// path executed when a domain user is disconnected from the domain
+			// pretend username is fullname
+			return username, nil
 		}
 		defer syscall.NetApiBufferFree(p)
 		i := (*syscall.UserInfo10)(unsafe.Pointer(p))