sim: rl78: clean up various warnings

A random grab bag of minor fixes to enable -Werror for this port.

Fix local prototypes for a bunch of functions (e.g. adding static).
Add missing includes for missing prototypes.
Move local variable decls from the middle of functions to the top
of the scope.
Fix a logic error when processing commands where p was reassigned
to cmd and then has its leading whitespace scanned a 2nd time.
Handle short reads with fread().