collection.cgi: Make sure to pass valid variable names to rrdgraph.

collection.cgi: Make sure to pass valid variable names to rrdgraph.

Variable names used with DEF, VDEF, etc. must be made up of the
characters "A-Z, a-z, 0-9, -, _". In meta_graph_generic_stack() variable
names have been generated from the instance name which may, however,
contain illegal characters. Now, all illegal characters will be replaced
with "_" and a number will be prepended to the variable name to make
sure the name is still unique.

Thanks to Joerg Jaspert for pointing this out and helpful debugging
information.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>