teximg: Make TeX handle preventing unsafe things; remove insufficient blacklist

teximg: Make TeX handle preventing unsafe things; remove insufficient blacklist

TeX has configuration options that prevent unsafe things like shell
escapes and insecure file reads/writes.  Turn all of them on.

teximg's regex-based blacklist does not suffice.  For instance:

[[!teximg code="""
\catcode`\%=0
%input{/etc/passwd}
"""]]

Remove the blacklist, since the TeX configuration options seal off the
underlying mechanisms more safely, and the blacklist blocks other TeX
commands that can prove useful.
(cherry picked from commit 9f75d3b1f3c43820cff9ce554601f64c60d72b14)

Conflicts:

	IkiWiki/Plugin/teximg.pm
	debian/changelog