This is more in line with how we generally handle user options for
commands to call. Later, we can get rid of the 'eval' call.
* lisp/textmodes/tex-mode.el (tex-view): Warn if tex-dvi-view-command is
set to an sexp instead of a string; say that this use is obsolete.
(tex-dvi-view-command): Change the default value to a string. Update
docstring to reflect the above obsoletion.
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