Check that plugin name is pure ASCII

We assume that plugin name is ASCII (at least by function which turn case down) but never really check it.

The assumption is quite visible in 929c2e06aae47f2dabf51b843ac84911de95bc7f

In early versions we see it in construct_options() (sql_plugn.cc):

my_casedn_str(&my_charset_latin1, plugin_name_ptr);

So the task is to make a dummy plugin (but having a dummy variable) with name contain enhanced characters.

Test how it behave now.

Then check in on plugin initialisation and issue an error.

name can be checked with help of following (thanks Bar for the idea):

my_repertoire_t my_string_repertoire(CHARSET_INFO *cs,

                                     const char *str, size_t len);

OK results:

  MY_REPERTOIRE_NONE=        0,

  MY_REPERTOIRE_ASCII=       1, /* Pure ASCII            U+0000..U+007F */