build: add build comments to core packages

The go/build package already recognizes
system-specific file names like

        mycode_darwin.go
        mycode_darwin_386.go
        mycode_386.s

However, it is also common to write files that
apply to multiple architectures, so a recent CL added
to go/build the ability to process comments
listing a set of conditions for building.  For example:

        // +build darwin freebsd openbsd/386

says that this file should be compiled only on
OS X, FreeBSD, or 32-bit x86 OpenBSD systems.

These conventions are not yet documented
(hence this long CL description).

This CL adds build comments to the multi-system
files in the core library, a step toward making it
possible to use go/build to build them.

With this change go/build can handle crypto/rand,
exec, net, path/filepath, os/user, and time.

os and syscall need additional adjustments.

R=golang-dev, r, gri, r, gustavo
CC=golang-dev
https://golang.org/cl/5011046

src/pkg/syscall/bpf_bsd.go

diff --git a/src/pkg/syscall/bpf_bsd.go b/src/pkg/syscall/bpf_bsd.go
index 1eac9a3..06a2953 100644
--- a/src/pkg/syscall/bpf_bsd.go
+++ b/src/pkg/syscall/bpf_bsd.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // Berkeley packet filter for BSD variants
 
 package syscall

src/pkg/syscall/exec_unix.go

diff --git a/src/pkg/syscall/exec_unix.go b/src/pkg/syscall/exec_unix.go
index 94f0756..2399c89 100644
--- a/src/pkg/syscall/exec_unix.go
+++ b/src/pkg/syscall/exec_unix.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Fork, exec, wait, etc.
 
 package syscall

src/pkg/syscall/route_bsd.go

diff --git a/src/pkg/syscall/route_bsd.go b/src/pkg/syscall/route_bsd.go
index 22a0a4f..f6b124b 100644
--- a/src/pkg/syscall/route_bsd.go
+++ b/src/pkg/syscall/route_bsd.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // Routing sockets and messages
 
 package syscall

src/pkg/syscall/sockcmsg_unix.go

diff --git a/src/pkg/syscall/sockcmsg_unix.go b/src/pkg/syscall/sockcmsg_unix.go
index b437560e..c9872ae 100644
--- a/src/pkg/syscall/sockcmsg_unix.go
+++ b/src/pkg/syscall/sockcmsg_unix.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 // Socket control messages
 
 package syscall

src/pkg/syscall/syscall_bsd.go

diff --git a/src/pkg/syscall/syscall_bsd.go b/src/pkg/syscall/syscall_bsd.go
index f59e8b1..59c6f45 100644
--- a/src/pkg/syscall/syscall_bsd.go
+++ b/src/pkg/syscall/syscall_bsd.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd openbsd
+
 // BSD system call wrappers shared by *BSD based systems
 // including OS X (Darwin) and FreeBSD.  Like the other
 // syscall_*.go files it is compiled as Go code but also

src/pkg/syscall/syscall_unix.go

diff --git a/src/pkg/syscall/syscall_unix.go b/src/pkg/syscall/syscall_unix.go
index c298b91..1590b6d 100644
--- a/src/pkg/syscall/syscall_unix.go
+++ b/src/pkg/syscall/syscall_unix.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd
+
 package syscall
 
 import (