config: use "long int" as required by my version of libconfig

Fixes:

config.c: In function ‘load_config’:
config.c:222: warning: passing argument 3 of ‘config_lookup_int’ from incompatible pointer type
/usr/include/libconfig.h:235: note: expected ‘long int *’ but argument is of type ‘int *’
config.c:245: warning: passing argument 3 of ‘config_lookup_int’ from incompatible pointer type
/usr/include/libconfig.h:235: note: expected ‘long int *’ but argument is of type ‘int *’
probability: use a faster binomial coefficient calculation

Computing with factorials is just horrendously slow, especially when
not factored; use the iterated multiplication definition instead.