misc: add zsh completion (using compctl)

R=golang-dev, rsc1, rsc
CC=golang-dev
https://golang.org/cl/2061042

misc/zsh/go

+# install in /etc/zsh/zshrc or your personal .zshrc
+
+# gc
+prefixes=(5 6 8)
+for p in $prefixes; do
+	compctl -g "*.${p}" ${p}l
+	compctl -g "*.go" ${p}g
+done
+
+# standard go tools
+compctl -g "*.go" gofmt
+
+# gccgo
+compctl -g "*.go" gccgo