Replace calls to `sprintf` and `strcpy` by the safer options `snprintf`
and `safe_strcpy` in the following directories:

- libmysqld
- mysys
- sql-common
- strings

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.