File descriptors and ulimits

[jsteemann]

On Linux and MacOS, the starter should try to increase the number of file descriptors available for itself and its sub-processes (i.e. arangod instances), by probing the current rlimits for "number of open files" and trying to increase them if they seem to low.

This is a nice-to-have improvement and does not seem necessary if the starter could display arangod startup errors, e.g. the ones that are due to too few file descriptors available.