zfs: pipe size: default to value of /proc/sys/fs/pipe-max-siz

Addition by @problame: move getPipeCapacityHint() into platform-specific
code. This has the added benefit of not recognizing the envvar as an
envconst on platform that do not support resizing pipes.  => won't show
up in (zrepl status --raw).Global.Envconst

fixes #424
cloes #449

zfs/zfs_pipe.go

@@ -7,10 +7,14 @@ import (
 	"sync"
 )
 
+func getPipeCapacityHint(envvar string) int {
+	return 0 // not supported
+}
+
 var zfsPipeCapacityNotSupported sync.Once
 
 func trySetPipeCapacity(p *os.File, capacity int) {
-	if debugEnabled {
+	if debugEnabled && capacity != 0 {
 		zfsPipeCapacityNotSupported.Do(func() {
 			debug("trySetPipeCapacity error: OS does not support setting pipe capacity")
 		})