Really enable core dumps in Linux

Really enable core dumps in Linux

commit e1f1ce68e1f685400a8c68bcec14018e3d1fc29d
Author: Rusty Russell <rusty@rustcorp.com.au>
Date:   Mon Sep 20 13:33:30 2010 +0930

    source3: dump core on Linux, even after seteuid/etc.

    The "dumpable" flag is reset on every call to set*uid, so we need to
    reset it to 1 in the signal handler itself.

    This code dates back to commit ac01fda2b97b 6 years ago :(

    Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit dc192787955542510724541cf23b4d8ed4cc99d4)