x/sys/unix: missing vmsplice on Linux

[mdlayher]

I'm doing some experiments and wanted to try out the vmsplice system call on Linux. unix.Splice already exists, but it appears that there is no unix.Vmsplice.

A cursory search reveals it in the "unimplemented" section of syscall_linux.go.

I implemented it myself, and I'd like to send a CL, unless there is an explicit reason this system call (and others) are not implemented in x/sys/unix.

[ianlancetaylor]

Sure, send a CL. Thanks.

[gopherbot]

CL https://golang.org/cl/37152 mentions this issue.