CHOP-959: Streamline processing for --graphicslevel switch.

Use map-to in cmd_line.xml to inform the command-line processor that the
target variable for --graphicslevel is RenderQualityPerformance.
That lets us eliminate clunky llappviewer.cpp switch from '0' to 0, etc.
Moreover, previous switch statement only accepted 0 - 3, whereas
LLFeatureManager::setGraphicsLevel() actually accepts 0 - 6. Introduce
LLFeatureManager::isValidGraphicsLevel() and use that to validate.
Replace switch statement in setGraphicsLevel() mapping int constants to string
literals with static vector of level names, using same data for mapping as for
validating level numbers.