WL#3868: startup parameters for INSTALL PLUGIN

currently, INSTALL PLUGIN always installs a plugin with all parameters having
default values. It doesn't always work, for example InnoDB needs to know
innodb_home_data_dir, otherwise it won't work.

We will support parameters, similar to command line or server variables, in the
INSTALL PLUGIN.

The syntax will be


  INSTALL PLUGIN pluginname SONAME "soname"
          [SET var1=val1[, var2=val2[, ...]]] ;


the parser will put these name=value pairs into an array. The sql_plugin.cc will
use my_getopt to parse this array and assign values to command-line parameters.

On mysqld shutdown these values are lost, as SET values always do. One should
use my.cnf for settings that should persist.

Also we could re-read my.cnf on every INSTALL PLUGIN and pick the relevant
options from there.