Fix issue #12: Standardize function naming style in core files

modified:   core/io.sh
   Standardized function headers.

modified:   core/mbs-lib.sh
   Standardized function headers.

   All core modules still need to have private / public functions renamed correctly.
   I'll file a separate issue since this is sure to break user packages right now.
   Ideally, this is where the standard user package would hide mbs internals a bit more, and user packages could interface with it at the user level.
   MBS is starting to resemble a tiny microkernel for encouraging you to do good script design.