edit.m: use current working directory, remove HOME

[Mike Miller]

I have prepared a new patch to make the edit command consistent wrt
where files are edited, please take a look at the attached and tell me
what you think.  The HOME variable is now no longer needed, a warning is
issued if the user tries to get or set HOME.  The current directory is
now used wherever HOME was used before, primarily when editing a system
m-file with no write privileges.

-- 
mike

edit-home.patch
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