but NOT from var_OptionParse().

Rationale: At a shallow level, this breaks the command line use badly.
At a deeper level: We still do security enforcement in
var_OptionParse(). In practice, the config chain strings are always
coming from (part of) the value of string configuration variable,
which is parsed by var_OptionParse(). Hence, as long as these variables
are all tagged as "unsafe", only trusted input can ever reach
config_ChainParse().

There are other (intractable?) issues with checking configuration chain
strings: First, we would also need to check the module name in front of
the {var1=val1,...} stuff. Second, some modules parse their
configuration chain manually, i.e. they don't call config_ChainParse()
and sometimes do not register their variables as configuration items
(e.g. the sout duplicate module).

If you have a super-duper idea on how to improve this, you're welcome,
but in the mean time...
Signed-off-by: Rémi Denis-Courmont <rem@videolan.org>
Vaguely-acked-by: Pierre d'Herbemont <pdherbemont@free.fr>