implementation)".

Starting from Vista/Server 2008 a native symlinking mechanism is
supported by Windows (through MKLINK command). Since starting from
5.6 we don't plan to support Windows XP/Server 2003 this makes MySQL
Server implementation of symbolic links for Windows (based on custom
.sym files) redundant. Therefore it is a good idea first to deprecate
and then completely remove code implementing custom symbolic links.

Note that having this custom implementation around is a bad idea not
only from code complexity view point, but it also creates performance
problems in some scenarios and is cause behind some bugs. 

This is first step towards custom symlink implementation removal. 
We start to emit deprecation warning to error log first time when
a table in such a .sym-linked database is used.

Test that checks that deprecation warning is emitted when
.sym-linked database is used for the first time is added.

Manual testing was done to check that native symlinks are
working as expected for linking databases.