config: Adjust process.env to immediately punt to POSIX

The uppercase letter / digit / underscore restriction is just for
"variables used by the utilities in the Shell and Utilities volume of
IEEE Std 1003.1-2001".

Copying over some POSIX wording and then linking to POSIX didn't seem
like much gain.  Just point people at POSIX and let them read about
the name=value definition, charset suggestions, etc. there.

Also link specifically to chapter 8 section 1 (instead of just chapter
8).

Rob Dolin had suggested "platform-appropriate" wording [1], but it
seems like Visual Studio 2015 supports an environment-variable array
with the same semantics [2], and providing an explicit
"platform-appropriate" wiggle seems like it's adding useless
complication.

[1]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-05-18-17.01.log.html#l-54
[2]: https://msdn.microsoft.com/en-us/library/431x4c1w.aspx

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

config.md

@@ -125,9 +125,7 @@ See links for details about [mountvol](http://ss64.com/nt/mountvol.html) and [Se
   * **`width`** (uint, REQUIRED)
 * **`cwd`** (string, REQUIRED) is the working directory that will be set for the executable.
   This value MUST be an absolute path.
-* **`env`** (array of strings, OPTIONAL) contains a list of variables that will be set in the process's environment prior to execution.
-  Elements in the array are specified as Strings in the form "KEY=value".
-  The left hand side MUST consist solely of letters, digits, and underscores `_` as outlined in [IEEE Std 1003.1-2001](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html).
+* **`env`** (array of strings, OPTIONAL) with the same semantics as [IEEE Std 1003.1-2001's `environ`][ieee-1003.1-2001-xbd-c8.1].
 * **`args`** (array of strings, REQUIRED) executable to launch and any flags as an array.
   The executable is the first element and MUST be available at the given path inside of the rootfs.
   If the executable path is not an absolute path then the search $PATH is interpreted to find the executable.
@@ -762,6 +760,7 @@ Here is a full example `config.json` for reference.
 
 [container-namespace]: glossary.md#container-namespace
 [go-environment]: https://golang.org/doc/install/source#environment
+[ieee-1003.1-2001-xbd-c8.1]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html#tag_08_01
 [runtime-namespace]: glossary.md#runtime-namespace
 [uts-namespace]: http://man7.org/linux/man-pages/man7/namespaces.7.html
 [mount.8-filesystem-independent]: http://man7.org/linux/man-pages/man8/mount.8.html#FILESYSTEM-INDEPENDENT_MOUNT%20OPTIONS