Removed duplicate code selecting which load average to use.
Fixed load average selection being in OSX/BSD part of if, making it not work
on non OSX/BSD systems.
Optimized actually getting the load average.
When variable POWERLEVEL9K_DIR_SHOW_WRITABLE is set to true, dir prompt
will have new state NOT_WRITABLE.
This state will be set when user doesn't have permission to write in
current working directory as in prompt dir_writable.
When mode that allows icons is set, LOCK_ICON is used.