oxenstored: add facilities to raise the max open fds uplimit

oxenstored: add facilities to raise the max open fds uplimit

To go beyond 1024 fds, we also need to raise the process limitation on max
open fds (usually defaults to 1024).

We need to know the system level max open fds so that we won't go above that.
Simply setting the limit to RLIM_INFINITY doesn't work on Linux 3.x (EPERM), a
patch on this went into the 2.x branch but not 3.x for some reason.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>