Prevent core dumps from being generated when memcached crashes

Given memcached uses breakpad to generate minidumps, which are cross-platform "core dumps", we should probably start disallowing core dump generation by default. We should especially protected against full core dumps containing the process heap from being generated, as those can be excessively large.

Options:

• We could setrlimit to disable core dump generation by default in newer versions, but allow for overriding this.
• Alternatively, we could just issue a warning in memcached.log when the coredump_filter will result in a core dump containing the entire process heap

Given breakpad has been reliable in the past, we could probably just disable core dumps by default.