[vile] Temp files in spell filter

[Decklin Foster]

The spell filter currently creates temporary files in vile's current
directory. This fails in any case where the directory is not writable by
the user. I've attached a patch to make the filter try TMPDIR from the
environment and fall back to /tmp. I have not searched the rest of the
code for other places that this might be a problem.

Note: Since the value of an environment variable could be long enough
to overflow I made open_tempfile take the buffer size as well and use
snprintf.

(I am totally unclear on why the spell filter needs temp files, but hey,
I just wanted things to work so I could send an email... :-)

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vile_spellflt_tmpdir.patch
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