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.