Avoid 'Too many open files' on Mac

Increase the number of file descriptors that the process can open
to the maximum allowed. By default, Mac OS X only allows 256 file
descriptors, which can easily be reached.

Change-Id: Ie4e5ab1051d8e3bf1cb39747f629360915a22d97
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>