(server crash)

Altering a table with fulltext index[es] which use
pluggable fulltext parser may cause server crash
in debug builds.

The problem was that ALTER TABLE code wrongly assigned
fulltext parser name.

Also fixed that altering a table with fulltext index[es]
leave stale fulltext parser locks, which prevent
fulltext parsers from being uninstalled after
ALTER TABLE.

mysql-test/include/have_simple_parser.inc:
  Added support for testing simple fulltext parser.
mysql-test/mysql-test-run.pl:
  Added support for testing simple fulltext parser.
mysql-test/r/fulltext_plugin.result:
  A test case for BUG#39746.
mysql-test/r/have_simple_parser.require:
  Added support for testing simple fulltext parser.
mysql-test/t/fulltext_plugin-master.opt:
  A test case for BUG#39746.
mysql-test/t/fulltext_plugin.test:
  A test case for BUG#39746.
sql/sql_table.cc:
  Fixed that alter table wrongly assigns fulltext parser
  name. parser_name member is only available during
  table creation. When we open existing table we must
  get parser_name from plugin_ref, which is handled
  by plugin_name() macro.
sql/table.cc:
  Moved code that releases fulltext parsers into
  free_table_share(). This fixes stale fulltext parser
  locks set by ALTER TABLE, which are preventing fulltext
  parsers from being uninstalled.