Make setting tex-dvi-view-command to an sexp obsolete

Make setting tex-dvi-view-command to an sexp obsolete

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set to an sexp instead of a string; say that this use is obsolete.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.

(cherry picked from commit 9b4af418ddc3328c8756f3ca21ba25f161c65b65)