train_common_threads.c:
* fix some bugs in multi-threaded training

html.c:
* various optimization and bug fixes

fnb_learn.c:
fnb_findtolearn.c:
* some speed ups by caching hashes
* various bug fixes
* update macros for modern glibc (should still work with older versions)

findtolearn_util.c:
bayes.c:
* more support for caching of hashes

fnb_learn.8.in:
fnb_findtolearn.8.in:
* clarify documentation

NOTE: This may be the last commit for a while.