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_freebsd_arm.go b/src/pkg/syscall/zerrors_freebsd_arm.go
index 29d92be..269f179 100644
--- a/src/pkg/syscall/zerrors_freebsd_arm.go
+++ b/src/pkg/syscall/zerrors_freebsd_arm.go
@@ -1094,6 +1094,9 @@
 	PARMRK                            = 0x8
 	PARODD                            = 0x2000
 	PENDIN                            = 0x20000000
+	PRIO_PGRP                         = 0x1
+	PRIO_PROCESS                      = 0x0
+	PRIO_USER                         = 0x2
 	PROT_EXEC                         = 0x4
 	PROT_NONE                         = 0x0
 	PROT_READ                         = 0x1