WL#2763: MySQL plugin interface: step 1

step 1 of WL#2761 - MySQL plugin interface.
It includes a minimal subset of WL#2761 that allows to create a usable plugin:

* C API
* mysql.plugin table

It does not include

* a possibility to add new syntax, status variables, command line options
* LOAD/UNLOAD/SHOW PLUGINS statements

Still, the implementor may decide to implement anything that is listed as "not
included",
if it'll be necessary to (see above) "create a usable plugin"

see WL#2761