unistd.h: Add a new pathconf variable for UF_HIDDEN/UF_SYSTEM

[rmacklem]

Summary

For the NFSv4 server to implement the "hidden" and
"system" attributes, it needs to know if UF_HIDDEN,
UF_SYSTEM are supported for the file.

This patch adds a new pathconf variable called
_PC_HAS_HIDDENSYSTEM to do that. The ZFS patch
will be handled separately as a OpenZFS pull request.

Although this pathconf variable may be queried
by applications using pathconf(2), the current
interface where chflags(2) returns EOPNOTSUPP
may still be used to check if the flags are set.

Test Plan

Tested with a kernel which has NFSv4 client and
server patches for UF_HIDDEN and UF_SYSTEM.

These NFSv4 patches will be committed once this patch
has been reviewed/committed.

[kib]

The var should be added to libsys/pathconf.2