x/sys/unix: add Solaris/Illumos ACL and extended attribute support

[dave-fl]

Would it be possible to get these calls added as part of Solaris/Illumos. Must use cgo to interface with file attributes otherwise. Some of these might exist, most don't. Most implementations seem to handle these via a syscall which we don't have in Solaris.

fgetattr,fsetattr,getattrat,setattrat
acl_get,facl_get,acl_set,facl_set, acl_totext, acl_fromtext,unlinkat
openat,fchownat,fstatat,futimesat,renameat, attropen, fdopendir, readdir, read, write

http://illumos.org/man/3lib/libc
http://illumos.org/man/3lib/libsec

What version of Go are you using (go version)?

1.10

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

amd64/solaris

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

Ability to change acls and extended attributes from syscall or os package.

What did you see instead?

Need to use cgo.

[gopherbot]

Change https://golang.org/cl/100555 mentions this issue: unix: add Fstatat on Solaris