util: minor tweaks to disable_core_dumps()

util: minor tweaks to disable_core_dumps()

First, let's rename it to disable_coredumps(), as in the rest of our
codebase we spell it "coredump" rather than "core_dump", so let's stick
to that.

However, also log about failures to turn off core dumpling on LOG_DEBUG,
because debug logging is always a good idea.