config: Require process.user.username on Windows

On POSIX (currently Linux and Solaris), `uid` and `gid` are
required. My preferred approach here is to make those optional and use
platform defaults [1,2]:

    If unset, the runtime will not attempt to manipulate the user ID
    (e.g. not calling setuid(2) or similar).

But the maintainer consensus is that they want those to be
explicitly required properties [3,4,5].

The Windows `username`, on the other hand, was optional, although the
default behavior is unclear.  I see no discussion in f9e48e0
(Windows: User struct changes, 2016-09-14, opencontainers#565) or its pull-request
discussion to suggest whether this was intentionally approved or not.
When I asked whether the optional-ness was intentional, Michael said
[6]:

  No, both should be made explicit unless there is something on
  windows that prohibits this.

So this commit is making that happen.

[1]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/DWdystx5X3A
[2]: opencontainers#417 (comment)
     Subject: Exposing platform defaults
     Date: Thu, 14 Jan 2016 15:36:26 -0800
     Message-ID: <20160114233625.GN6362@odin.tremily.us>
[3]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-05-04-17.00.log.html#l-44
[4]: opencontainers#417 (comment)
[5]: opencontainers#417 (comment)
[6]: opencontainers#618 (comment)

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

config.md

@@ -259,7 +259,7 @@ _Note: symbolic name for uid and gid, such as uname and gname respectively, are
 
 For Windows based systems the user structure has the following fields:
 
-* **`username`** (string, OPTIONAL) specifies the user name for the process.
+* **`username`** (string, REQUIRED) specifies the user name for the process.
 
 ### Example (Windows)