Using positional and vararg arguments for BIFs, it's not possible to do
proper runtime type checking on them as discussed in #2425. The bifcl produced
code unconditionally attempts to convert the positional arguments to StringVals,
but nothing ever type checks them. Instead of improving the vararg support in
Zeek script and bifcl, align cat_sep() with fmt() in making it fully vararg
and do implement type checks by hand.
With this change, passing wrong types for the separator and default argument
isn't a fatal error anymore and the error messages are also more descriptive.
It's a bit of a crutch working around varargs limitations.
Fixes#2425
