WL#9014: Deprecate and remove mysql_shutdown()

We have decided to deprecate COM_SHUTDOWN and the C API function mysql_shutdown().

However, while WL#6784 mentions
 * Creating SHUTDOWN SQL statement
 * Deprecating COM_SHUTDOWN
It does not mention mysql_shutdown(). 

Hence the proposal in the WL is to deprecate mysql_shutdown() in 5.7 and remove
it in 5.8.

User Documentation
==================

Documentation work consists of *removing* the mysql_shutdown() section, so there 
is not much to list here. Will mention the removal in the 5.8.0 release notes, 
which are not yet public.

F1: remove all traces of mysql_shutdown() in 5.8
F2: remove all traces of COM_SHUTDOWN in 5.8.
F3: keep the ID for COM_SHUTDOWN for binary compatibility, but rename it to
COM_DEPRECATED_1