Undocumented change: Core dumps can now be enabled dynamically

Hi,

In MariaDB 10.1.23 and 10.2.6, core dumps can be enabled by changing the process limit:

prlimit --core=unlimited: --pid `pidof mysqld`

In MariaDB 10.1.19 this required a restart.

Way to test:
1. Install the repository
2. Install 10.1.23
3. change the process limit
4. kill -SIGSEGV `pidof mysqld`
5. Observer there is a core dump

Do the same with MariaDB 10.1.19 and observe there is no core dump.

This change was undocumented but it is very useful desired behaviour. Please add this change to the release notes and/or find out what changed it.