core: set dumpable flag after setuid

core: set dumpable flag after setuid

When ceph-* drops drops privileges via setuid, core dumps are no longer
generated because its DUMPABLE flag is cleared. We have to manually
turn that back on.

From prctl(2):

      Normally, this flag is set to 1.  However, it is reset to the current value contained in the file /proc/sys/fs/suid_dumpable (which by default has the value 0), in the fol‐
      lowing circumstances:

      *  The process's effective user or group ID is changed.

      *  The process's filesystem user or group ID is changed (see credentials(7)).

      *  The process executes (execve(2)) a set-user-ID or set-group-ID program, or a program that has capabilities (see capabilities(7)).

Fixes: http://tracker.ceph.com/issues/17650
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>