posix_fallocate(2): move unsupported case to EOPNOTSUPP

[kevans]

Summary

ZFS has since been changed to use EOPNOTSUPP instead of EINVAL, and
fusefs/nfs are following suit. POSIX Issue 8 had also made this move,
so it makes sense for us to standardize on EOPNOTSUPP.

[imp]

Worth an issue 8 note in the history standards section?

[kib]

In D53537#1222227, @imp wrote:

Worth an issue 8 note in the history standards section?

Might be, indeed, add such note plus note that previous versions of zfs returned EINVAL.

[kevans]

Describe the switch a little more in HISTORY and update the STANDARDS; the
notable changes since -2004 are this different error return and clarification
of EFBIG to surface if the file size limit is exceeded (with generation of a
SIGXFSZ).