unistd.h: Add new Solaris compatible pathconf names

Solaris software can use _PC_XATTR_ENABLED and
_PC_XATTR_EXISTS, which are semantically the same
as _PC_NAMEDATTR_ENABLED and _PC_HAS_NAMEDATTR.

Add these pathconf names to unistd.h for Solaris compatibility.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51546
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")