os/signal: move to exp/signal.

Fixes #2816.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5609048

doc/go1.html

@@ -558,7 +558,7 @@ Several packages have moved under <code>exp</code> at the time of Go 1's release
 <ul>
 <li><code>ebnf</code></li>
 <li><code>go/types</code></li>
-<li><code>http/spdy</code></li>
+<li><code>os/signal</code></li>
 </ul>
 
 <p>

doc/go1.tmpl

@@ -482,7 +482,7 @@ Several packages have moved under <code>exp</code> at the time of Go 1's release
 <ul>
 <li><code>ebnf</code></li>
 <li><code>go/types</code></li>
-<li><code>http/spdy</code></li>
+<li><code>os/signal</code></li>
 </ul>
 
 <p>