Problem: --symbolic-links is problematic, and a source of recent
bug reports. Since symbolic links allow MySQL to write data to any effective
location on the operating system, it creates two issues:

1) MySQL may be tricked into writing to locations it is not supposed to

2) MySQL may write to a location that has privileges that are too
   loose, and may be tampered with by other users on the operating
   system.

Fix:
1) Change the compiled default to OFF (aligning with the defacto default)
2) Issue deprecation warning if the option is used.