syscall: add {Set,Get}priority for Linux, and consistency tests

R=golang-dev, iant, bradfitz, dave
CC=golang-dev
https://golang.org/cl/7430044
diff --git a/src/pkg/syscall/zerrors_netbsd_386.go b/src/pkg/syscall/zerrors_netbsd_386.go
index c0ef19f..9b93f5a1 100644
--- a/src/pkg/syscall/zerrors_netbsd_386.go
+++ b/src/pkg/syscall/zerrors_netbsd_386.go
@@ -1040,6 +1040,9 @@
 	PARODD                            = 0x2000
 	PENDIN                            = 0x20000000
 	PRI_IOFLUSH                       = 0x7c
+	PRIO_PGRP                         = 0x1
+	PRIO_PROCESS                      = 0x0
+	PRIO_USER                         = 0x2
 	RLIMIT_AS                         = 0xa
 	RLIMIT_CORE                       = 0x4
 	RLIMIT_CPU                        = 0x0