Tokens that begin with two underscores __* or an underscore and an uppercase letter _X* are reserved,
so we should avoid those for our own include guards. The standard I'm settling on for foo.h is FOO_H_.
Functions display_msg() and check_sync() are only used in numbers.c, so they
can be static.
Remove inclusions of <numbers.h> from .c files that don't need it.
