This patch implements "permanent" load option for
plugins as specified by WL#5341.

mysql-test/r/plugin_load_option.result:
  A test case for WL#5341.
mysql-test/t/plugin_load_option-master.opt:
  A test case for WL#5341.
mysql-test/t/plugin_load_option.test:
  A test case for WL#5341.
sql/share/errmsg-utf8.txt:
  An error message for WL#5341.
sql/sql_plugin.cc:
  Added FORCE_PLUS_PERMANENT plugin load option.
sql/sql_plugin.h:
  Expose and use plugin load option instead of
  is_mandatory flag. This is a requirement for
  to-be-implemented WL5496.