unix: add Uname on FreeBSD

FreeBSD doesn't have a uname syscall but the information can be
retrieved using sysctls the same way as on Darwin.

Change-Id: I824c42490d1feed3f1ad3823427c01dd3e5ea3c1
Reviewed-on: https://go-review.googlesource.com/79918
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zerrors_freebsd_amd64.go

@@ -351,6 +351,8 @@ const (
 	CSTOP                          = 0x13
 	CSTOPB                         = 0x400
 	CSUSP                          = 0x1a
+	CTL_HW                         = 0x6
+	CTL_KERN                       = 0x1
 	CTL_MAXNAME                    = 0x18
 	CTL_NET                        = 0x4
 	DLT_A429                       = 0xb8
@@ -608,6 +610,7 @@ const (
 	F_UNLCKSYS                     = 0x4
 	F_WRLCK                        = 0x3
 	HUPCL                          = 0x4000
+	HW_MACHINE                     = 0x1
 	ICANON                         = 0x100
 	ICMP6_FILTER                   = 0x12
 	ICRNL                          = 0x100
@@ -944,6 +947,10 @@ const (
 	IXANY                          = 0x800
 	IXOFF                          = 0x400
 	IXON                           = 0x200
+	KERN_HOSTNAME                  = 0xa
+	KERN_OSRELEASE                 = 0x2
+	KERN_OSTYPE                    = 0x1
+	KERN_VERSION                   = 0x4
 	LOCK_EX                        = 0x2
 	LOCK_NB                        = 0x4
 	LOCK_SH                        = 0x1