Make server compile with C/C client headers

The C/C has a large number of incompatibilities with existing libmysql headers, thus server does not compile with it. In order to really get rid of libmysql, which was planned already for 10.2 but now has to be postponed to 10.3 due to this, C/C should change to adhere to existing interfaces. Major incompatibilities are missing VIO and NET structures, there is more, which I do not remember now.

To reproduce , remove this file. https://github.com/MariaDB/server/blob/10.2/include/mysql.h
and replace with C/C's (set INCLUDE_DIRECTORIES inside sql directory to libmariadb/include)