NetBSD support

[edsrzf]

Thanks for looking at this!

Since the two different msync functions are basically the same except for the constants, I think it would be better to actually keep things closer to how they were and just define the constants. So msync_netbsd.go can have:

const _SYS_MSYNC = 277
const _MS_SYNC = 0x04

And msync_unix.go:

const _SYS_MSYNC = syscall.SYS_MSYNC
const _MS_SYNC = syscall.MS_SYNC

Then change flush in mmap_unix.go to use those constants.

[artbin]

I changed the code

[edsrzf]

Thanks!