Fix parsing of invalid plugin enum option value.

Previous patch to fix plugin enum option parsing on big-endian introduced another bug due
to incorrect comparison of unsigned value. This would cause an incorrect value to be
parsed as value 0.

See also MySQL Bug#41010 and Bug#32034.


mysql-test/mysql-test-run.pl:
  Add a facility for test case to run the mysqld binary (to test that invalid startup options
  are rejected correctly).
mysql-test/r/mysqld_option_err.result:
  Add a test case to check that invalid startup options for mysqld are rejected.
  This is needed to test MBug#423035.
  Also add a few other similar tests, as this was completely untested before this patch.
mysql-test/t/mysqld_option_err.test:
  Add a test case to check that invalid startup options for mysqld are rejected.
  This is needed to test MBug#423035.
  Also add a few other similar tests, as this was completely untested before this patch.
mysys/my_getopt.c:
  Fix parsing of invalid plugin enum option value.