fix format string mismatch

Use the z modifier (e.g., %zi instead of %i or %li) to print sizes.
This is a C99 standard format character supported by rtapi_vsnprintf
Add header guards to certain files that lacked them
drop conditional defines for kernels older than 2.6.24

Ubuntu Hardy is 2.6.24.  This kernel was released over 5 years ago.
Don't include unneeded headers
Use RTAPI APIs to declare module parameters
Add new rtapi headers

.. which wrap existing kernel APIs in the rtapi_ namespace, so that we
can later implement them in userspace too
Add additional APIs to rtapi_string.h
Partially implement some new rtapi_ APIs for userspace
Document new rtapi headers
Use new rtapi headers and rtapi_ APIs
sim: implement EXPORT_SYMBOL_GPL

.. as the same as EXPORT_SYMBOL (i.e., we cannot yet produce a diagnostic
when we encounter a GPL mismatch)
hostmot2: make it build against rtapi_ APIs