properly escape in TeXFormat

See message "unescaped underscore" on fricas-devel (15-Apr-2014).
https://groups.google.com/d/msg/fricas-devel/lCgu8gvVOIM/T2-HyrnpsXcJ

Underscores should be escaped by a backslash like this: \_.
These are the problems that are resolved by this patch.

(5) -> )set output algebra off
(5) -> )set output tex on
(5) -> escape()

$$
_
\leqno(5)
$$

(6) -> 1.2345678901234567890123

$$
1.2345678901_23456789
\leqno(6)
$$

(1) -> "a_b_"\"

   (1)  "a_b"\"
$$
\mbox{\tt "a_b"\"}
\leqno(1)
$$

After the patch.

(1) -> )set output algebra off
(1) -> )set output tex on
(1) -> escape()

$$
\_
\leqno(1)
$$

(2) -> 1.2345678901234567890123

$$
1.2345678901\_23456789
\leqno(2)
$$

(3) -> "a_b_"\"

$$
\verb#"a_b"\"#
\leqno(3)
$$

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1766 b0c55286-4f34-0410-a049-a1e7e93b0762