Don't require users targetting Hyper-V to set an empty object ("root":
{}).  This also avoids confusion about whether you can set
root.readonly without setting root.path (you can't).

Move the relative, absolute, and rootfs bits into a POSIX paragraph,
because they don't apply to Windows where the value MUST be a volume
GUID path (since 2283e63, Windows: Remove Sandbox, additional tweaks,
2017-05-23, opencontainers#849).

We don't need the "for Windows Server containers" condition on volume
GUID paths, because with this commit that condition is already applied
at the 'root' level and the Hyper-V case has already been handled
there.

Signed-off-by: W. Trevor King <wking@tremily.us>

As requested by Michael [1], this reduces the number of changes
required if BSD or some such is added to the config.

This change leaves a bit of a gap between the platforms listed in
spec.md, since readers have to figure out on their own that the POSIX
properties apply to the linux and solaris platforms (and potentially
other future platforms).  But Michael felt like it's ok to leave that
gap, at least for now [2].

I've used "On POSIX platforms" and similar (instead of "On POSIX")
because, as Tianon points out [3], there is some space between
POSIX-the-spec and platforms which implement it.

[1]: opencontainers#838 (comment)
[2]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2017/opencontainers.2017-06-01-17.41.log.html#l-60
[3]: opencontainers#838 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>