#164 exec.n & tclvars.n changes

[David Gravereaux]

I like good docs. Here's a submission for some more.
I don't know how the formatting looks, though.

I added a note in tclvars.n about windows environment
variables not being allowed to be empty.

I added a note in exec,n about how console
applications that require a console and are run from
wish will fail as they're launched detached.

[David Gravereaux]

[Jeffrey Hobbs]

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Added to 8.4, but without the change that .cmd is also
searched for on NT. The tclWinPipe.c:ApplicationType
function clearly only adds .com, .exe and .bat.

[David Gravereaux]

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ok, sorry about slipping that one in. I have a different
tclWinPipe.c:ApplicationType I'm playing around with.