sql_plugin.h currenctly defines a macro "SHOW_always_last" which adds
extra server-only enum values to the "enum_mysql_show_type" externally
visible enum defined in plugin.h. This requires all server code to include
sql_plugin.h before plugin.h to get the correct enum definition. This is
both bug-prone and problematic for refactoring.

This patch removes SHOW_always_last and instead puts the server-only
values in separate server header file which is included in plugin.h protected
by #ifdef MYSQL_SERVER. This removes the awkward dependency between
sql_plugin.h and plugin.h.

By removing the bogus SHOW_always_last enum from the plugin.h
defintion, this patch breaks ABI compatibility. Hence the patch also updates
the required .pp files. As we have already bumped the libmysqlclient
version, this should not be a problem (See Bug#19729266).