Fix daemon_dumps_core on Linux when started as root

Under Linux, systems calls that manipulate UIDs and GIDs reset the
process dumpable flag to 0. When Naemon is started as root it drops
privileges using setuid/setgid, preventing core dumps from being
produced regardless of the value of daemon_dumps_core. This patch fixes
that by calling prctl() in drop_privileges().

Signed-off-by: Adam James <adam.james@transitiv.co.uk>