Add support for ENV of the form: ENV name=value ...

still supports the old form: ENV name value.

I have mixed feelings about this.  While it works and is kind of cool to be able to do:
ENV abc=def foo="feed the dog" hungry=eat\ a\ pizza late="go to bed 'now' I said"
and each key=value is parsed like you'd expect it to be on the command line, the ability to do more than
one ENV at a time really should be done under some kind of batching mechanism.  But, of course,
that's not there yet and we're not sure when it will be.  So, unlike the RUN command where we can
concat things together with &&'s, the ENV command doesn't have that option - not without this feature.

Closes #2333

Signed-off-by: Doug Davis <dug@us.ibm.com>
actually I was about to add it (just for you  :-) ) but then I realized, isn't the ... in there good enough since it means "repeat the last thing" ?  If I added:
ENV <key1>=<value1> <key2=value2> ...
people might wonder what the difference is between that line and the previous one.  And are we really saying we have a special syntax where we specify two or more and not just one or more?

At least my interpretation of that chunk of docs is that its not a sample but more like a pseudo-schema of what's expected.  But I could be wrong.