multi-arg exec and nested ${pref1:${pref2}}

multi-arg exec and nested ${pref1:${pref2}}

exec "command" "several args separated by spaces in 1 string".
I could not figure out how to nicely deal with multiple args in the parser,
so I just left it as DGL_CMD1, and do the separation manually.
Does the separation before dgl_format_str substitutions, to prevent
relying on potentially user-supplied input not having extra spaces embedded.

nested preferences - this just fixes a bug where we assumed the token
ended on the first }.