adapted to nacl (go playground)

[hedzr]

@sevlyar here is a patch for NaCL and go playground.

This patch had been built ok with:

GOARCH=amd64p32 GOOS=nacl go build examples/cmd/gd-simple/simple.go
for GOOS in windows linux darwin solaris; do go build examples/cmd/gd-simple/simple.go; done

All changes are:

• split syscall codes from signal.go as signal_init*.go
• add syscall_dup_nacl.go

💃