scripts/realtime.in:  fix broken kthreads module loading
configure.in:  fix documentation at beginning of file
configure.in:  fix broken arch test

oops
realtime.in:  fix module unloading
Makefile:  UB: cp -l kbuild breaks if the files already exist; add -f
configure.in:  add THREAD_FLAVOR_NAME macro; unify RTAPI_COMMON.c