unix: add some FreeBSD Capsicum support.

Change-Id: Ibb1426ebd18d27ee9f144af0fd8c94b08c06f7d7
Reviewed-on: https://go-review.googlesource.com/47290
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/mkerrors.sh

@@ -75,6 +75,7 @@ includes_DragonFly='
 '
 
 includes_FreeBSD='
+#include <sys/capability.h>
 #include <sys/param.h>
 #include <sys/types.h>
 #include <sys/event.h>
@@ -401,6 +402,7 @@ ccflags="$@"
 		$2 ~ /^(BPF|DLT)_/ ||
 		$2 ~ /^CLOCK_/ ||
 		$2 ~ /^CAN_/ ||
+		$2 ~ /^CAP_/ ||
 		$2 ~ /^ALG_/ ||
 		$2 ~ /^FS_(POLICY_FLAGS|KEY_DESC|ENCRYPTION_MODE|[A-Z0-9_]+_KEY_SIZE|IOC_(GET|SET)_ENCRYPTION)/ ||
 		$2 ~ /^GRND_/ ||