Implement volume limiter feature

Add a "volume limit" parameter to the configuration file. Each time when
setvol() is excuted, Rockbox will check if the
global_settings.volume value larger than global_settings.volume_limit.
If larger, take the value of volume_limit instead. The volume_limit
argument shares the same maximum and minimum value with volume argument.

Change-Id: I8ca9bc6ea6e617b48fc346aae5f2a0a1d259ae36