Fix narrowing conversion of ‘-1’ from ‘int’ to ‘rlim_t‘
Fix narrowing conversion of ‘-1’ from ‘int’ to ‘rlim_t‘

We were using -1 as a sentinel to determine if we had computed the
file descriptor limit, but rlim_t is unsigned. Also, RLIM_INFINITY
is typically -1 as well, so the choice of -1 as a sentinel was
probably not a good idea anyways.

Closes issue #19.

torarnv/sparsebundlefs,torarnv/sparsebundlefs