Fix get_maxfds() in jevents

If RLIM_INFINITY == -1ULL (such as on macOS) the min() call will result
in a value of less than 1 being returned. This causes nftw() to fail
with EINVAL.

While touching this file also fix includes to work on Linux/macOS and don't
declare snprintf since it may have different attributes in the system
headers there.

Reviewed By: mmacy
Approved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D17682