syscall: match linux Setsid function signature to darwin

SETSID does return an errno - any reason why it has been done this
way in zsyscall_linux_* ? Otherwise it should be the same as darwin.

From SETSID(2) on my Linux box:

ERRORS
       On error, -1 is returned, and errno is set.

Fixes #730

R=rsc
CC=golang-dev
https://golang.org/cl/878047