sympref: Document the inherent insecurity of some IPC mechanisms (e.g. system)

[alexvong243f]

Some of the IPC mechanisms currently in use are inherently insecure, such as system. We should at least document these caveats and consider deprecating them after the pythonic interface is ready for production use.

Quoting the man page of the C function system,

Any user input that is employed as part of command should be carefully sanitized, to ensure that unexpected shell commands or command options are not executed. Such risks are especially grave when using system() from a privileged program.

See also discussion in #1140.

[cbm755]

Just to keep our priorities in order: our upstream makes use of eval [1]. Not that we should ignore security stuff at our end, of course.

[1] sympy/sympy#10805

[alexvong243f]

Sure, we can deal with this later.