Don't set the nofile rlimit if the max is infinity

[xSetech]

• Subtraction occurred on a negative-integer (enum for infinity) resulting in
  an EINVAL from setrlimit().

• On macOS, setting the limit to RLIM_INFINITY is an error (see man setrlimit).

[coveralls]

Coverage decreased (-0.06%) to 74.212% when pulling 851a05e on xSetech:nofile into 37bfc5a on worr:master.

[worr]

If you force push, please leave a comment, since github doesn't give me notifications for those.

That said, while the code looks good, we raise the nofile for a reason - specifically because we're file descriptor hungry, what with having all those simultaneous ssh connections. It's probably better to detect infinity, and set the soft limit to that rather than to short-change the user on file descriptors.