RFC: _PC_HAS_HIDDENSYSTEM rename

Hi

There are three flags UF_HIDDEN, UF_SYSTEM and UF_ARCHIVE
defined in sys/stat.h (their origin is in MS-DOS). If a file system implements
any of these flags, they implement all three of them, afaik.

Commit afd5bc6309 (in main) defined _PC_HAS_HIDDENSYSTEM
to allow pathconf to indicate if a file system supports the first two of
these.
I had ignored the third, since it is listed as "deprecated" in RFC-8881.
It now turns out that the Windows NFSv4.1 client folk want support
for UF_ARCHIVE and consider that it should not be deprecated.

I do not think adding a separate _PC_HAS_ARCHIVE pathconf
name is useful, since it is supported when _PC_HAS_HIDDENSYSTEM
is returned non-zero.

However, the name _PC_HAS_HIDDENSYSTEM is misleading,
so what do others think I should do?
- Add an alias for _PC_HAS_HIDDENSYSTEM called something
 like _PC_HAS_HIDSYSARCH or _PC_HAS_HIDDENSYSTEMARCHIVE?
(Since _PC_HAS_HIDDENSYSTEM is now in the OpenZFS code, it
needs to remain in unistd.h, at least for now.)
Or
- Just leave the name as is and document it correctly in pathconf(2)?

Thanks for any comments, rick