a better approach for getting device arguments from dev.args when it is a list of lists

normally the device arguments should not be a list of lists, so we check sapply(dargs, is.list) and decide whether we want dargs[[dev]], instead of checking if names(dargs) contain the device names

this make it possible to use dev='pdf', dev.args=list(pdf = list(useDingbats=FALSE)); when dev='png' (or tikz, ...), the correct arguments will be passed to the device, otherwise we have problems, e.g. rstudio/rmarkdown@1029bd94ba2c35
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