bmake exports disallowed environment variables

[Simon J. Gerraty]

Hi Chris,

>bmake appears to export ${.MAKE.LEVEL} into the environment, which sh
>doesn't support, due to the leading '.'.   Normally this is ignored,

Yes, though env(1) does allow it.  The leading '.' was deliberately chosen
to reduce the risk of this being picked up from user environment.
That is, if make finds .MAKE.LEVEL in its environment, then it "trusts"
it - either an ancestor was make (rather than say gmake), or the user
"must know what they are doing".

In short, I'd like to leave it as is.