syscall, os: use pipe2 syscall on FreeBSD instead of pipe
The pipe2 syscall exists in all officially supported FreeBSD
versions: 10, 11 and future 12.
The pipe syscall no longer exists in 11 and 12. To build and
run Go on these versions, kernel needs COMPAT_FREEBSD10 option.
Based on Gleb Smirnoff's https://golang.org/cl/38422