[Review Request] Constrain get_next_dir() to directories below global_settings.start_directory. (#182)

[Nick Peskett]

From the commit description;

If the user has set "Start File Browser Here" (config.cfg: start 
directory) to anything other than root and "Auto-Change Directory" is 
set to "Yes" or "Random", the directory returned when an auto change is 
required will be constrained to the value of "start directory" or below.

http://gerrit.rockbox.org/r/#/c/182/