Increase volume control granularity in startup and fppd.

fppxml.php already uses a float when setting the volume percentage,
so the increment/decrement by 1 in the UI equates to a 0.5% bump
up or down, but the startup script and fppd still use integers
when setting the volume.  This commit changes these two locations
over to use a float also, so the volume after boot should match
exactly what it does when you change it through the UI.