unix: add Uname on dragonfly

The Utsname members are only 32 bytes on Dragonfly and the syscall
returns ENOMEM in case the value is longer than that. Like uname(3)
on Dragonfly, handle this case gracefully and just truncate the
value.

Change-Id: If617af1b6831cff6d4245f498dad9f264b8fd118
Reviewed-on: https://go-review.googlesource.com/82155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/zerrors_dragonfly_amd64.go

@@ -168,6 +168,8 @@ const (
 	CSTOP                             = 0x13
 	CSTOPB                            = 0x400
 	CSUSP                             = 0x1a
+	CTL_HW                            = 0x6
+	CTL_KERN                          = 0x1
 	CTL_MAXNAME                       = 0xc
 	CTL_NET                           = 0x4
 	DLT_A429                          = 0xb8
@@ -353,6 +355,7 @@ const (
 	F_UNLCK                           = 0x2
 	F_WRLCK                           = 0x3
 	HUPCL                             = 0x4000
+	HW_MACHINE                        = 0x1
 	ICANON                            = 0x100
 	ICMP6_FILTER                      = 0x12
 	ICRNL                             = 0x100
@@ -835,6 +838,10 @@ 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