x/sys/unix: missing flags support for Setxattr on freebsd.

[hirochachacha]

Currently, flags parameter is ignored.
We can implement XATTR_CREATE and XATTR_REPLACE for this.

[ayang64]

@hirochachacha, @tklauser -- has setxattr() ever been supported by FreeBSD? currently, there's no syscall.Setxattr() available if your GOOS is freebsd.

[tklauser]

@ayang64 this issue is about unix.Setxattr in the golang.org/x/sys/unix package which is available for GOOS=freebsd: https://github.com/golang/sys/blob/5a964db013201115fcba5c3d31ade965d0969335/unix/xattr_bsd.go#L100-L114

[ericlagergren]

@tklauser Do you mean adding dummy flags for GOOS=freebsd?