syscall: mark ECONNRESET, ECONNABORTED as temporary network errors

Fixes #6163.

LGTM=adg
R=golang-codereviews, adg, dvyukov
CC=golang-codereviews
https://golang.org/cl/141600043

src/syscall/syscall_unix.go

@@ -109,7 +109,7 @@ func (e Errno) Error() string {
 }
 
 func (e Errno) Temporary() bool {
-	return e == EINTR || e == EMFILE || e.Timeout()
+	return e == EINTR || e == EMFILE || e == ECONNRESET || e == ECONNABORTED || e.Timeout()
 }
 
 func (e Errno) Timeout() bool {