Support for AIX extended attributes?

[vmlemon]

On AIX, it is possible to manipulate extended attributes, using the setea(), getea(), listea(), and removeea() APIs, documented at https://www.ibm.com/docs/en/aix/7.3?topic=s-setea-subroutine.

These are also used heavily, by the AIX-compatible PASE environment, on IBM i (https://www.ibm.com/docs/en/i/7.3?topic=concepts-extended-attributes), to handle metadata such as codepages, file types, and descriptive metadata, and presumably also to store AIXC-style access control list metadata, given that pax is able to archive them, when xattrs are enabled, so it may be useful, to implement support, to enable utilities written in Go, such as Restic, to be able to work with them.

[kuba--]

@vmlemon - I do not have access to any AIX machine (it should be available on IBM cloud), so my implementation would be kind of blind (we need to think about some testing, ideally CI tests).
But any PR is very welcome!