[WIP]Implement volume limiter feature

Add a "volume limit" parameter to the configuration file. The maximum
value of volume setting in sound menu will be limited to the same as
volume limit. Also 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 preference values
with volume argument. Also implement a AUDIOHW_SETTING_CLN() macro to
clone a AUDIOHW_SETTING item from an existing one.

Change-Id: I8ca9bc6ea6e617b48fc346aae5f2a0a1d259ae36