Cleaned up the warnings produced by Clang.

*Minor changes in many files to remove warnings.
*Removed inline attribute from port_io declarations as it produced
a warning (?).
*Fixed broken function names in the GDT header.
*Removed unused <stdbool.h> header inclusion where it
wasn't no longer needed.
*Other minor fixes.
*Updated the 'clean' target in the Makefile.