SL-11049: Notice explicit --grid command-line switch and honor it.

LLPanelLogin's constructor checks both LLGridManager::getGrid() and
LLStartUp::getStartSLURL(). But by the time we get there, we've blurred the
distinction between explicit command-line arguments and defaults left over
from a previous run. Of course, if the grid implied by getStartSLURL() is the
same as the getGrid(), the distinction is irrelevant.

But if they differ, up until now, getStartSLURL() has always "won" -- even
when getGrid() was set by an explicit --grid switch whereas getStartSLURL()
was only left over from a previous run.

Notice that case and try to avoid overriding the explicitly-specified grid
with the grid from the default SLURL.