Increment the rlimit_nofile for all sandbox modes

Currently, we only increment the rlimit_nofile with the user-supplied value within sandbox modes 'chroot' and 'namespace'. However, in the 'namespace' mode, we don't honor '--rlimit-nofile=0' and we increase the value to the maximum, as in the default case.

This commit try to increase the rlimit_nofile for all modes, honoring '--rlimit-nofile=0'. Additionally, it leaves the limit unchanged if the limit is greater than 1M. Also, if everything fails the hard limit is used.

Closes #37 (closed)