Keep track of FD used for UST applications (v2)

Allow to keep 25% of file descriptors reserved for commands/kernel
tracing/internal communication within the sessiond by limiting
applications to 75% of the available file descriptors. This ensures
traced applications cannot cause a sessiond denial of service.

Changelog since v1:
- Add missing fd-limit.h

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>