merge of NFSv4.2 support into head/current

Hi,

I have completed development and a testing cycle for the NFSv4.2 (RFC-7862)
code in base/projects/nfsv42 on subversion.

NFSv4.2 is a minor revision to NFSv4.1 and adds support for the following optional
features:
- lseek(SEEK_DATA/SEEK_HOLE)
- posix_fallocate()
- posix_fadvise(POSIX_FADV_WILLNEED/POSIX_FADV_DONTNEED)
- Server side copy of byte ranges between two files on the same NFS mount
  point when the copy_file_range(2) syscall is used.
- Extended attribute support as specified by RFC-8276.
(There are some other optional features, but I do not intend to implement those
 at this time.)

Although this patch is fairly large, it should not affect the other versions of
NFS.

If anyone would like to do testing of it now, all you need is a fairly current
FreeBSD-current system, with the kernel replaced by one built from the
sources found in the above projects area. (And then you specify "minorversion=2"
as a mount option.

If anyone sees a problem with merging this code into head/current over
the next few weeks, please let me know.

Thanks, rick