MINOR: init: add a "set-dumpable" global directive to enable core dumps

MINOR: init: add a "set-dumpable" global directive to enable core dumps

It's always a pain to get a core dump when enabling user/group setting
(which disables the dumpable flag on Linux), when using a chroot and/or
when haproxy is started by a service management tool which requires
complex operations to just raise the core dump limit.

This patch introduces a new "set-dumpable" global directive to work
around these troubles by doing the following :

  - remove file size limits     (equivalent of ulimit -f unlimited)
  - remove core size limits     (equivalent of ulimit -c unlimited)
  - mark the process dumpable again (equivalent of suid_dumpable=1)

Some of these will depend on the operating system. This way it becomes
much easier to retrieve a core file. Temporarily moving the chroot to
a user-writable place generally enough.