The --enable_prepare_warnings allows to not discard warnings from autorepair
of crashed table in --ps-protocol mode.

Use this to properly fix the parts.partition_recover_myisam and
maria.maria-recover tests.

Add a test case for the new feature. This also adds missing test coverage
for the case where the same warning is thrown in both prepare and execute
phase.


client/mysqltest.cc:
  Implement new commands --enable-prepare_warnings and --disable_prepare_warnings.
mysql-test/r/mysqltest_ps.result:
  Add test case for new --enable_prepare_warning mysqltest command.
mysql-test/suite/maria/t/maria-recover.test:
  Better fix of test case using new --enable_prepare_warnings command.
mysql-test/suite/parts/t/partition_recover_myisam.test:
  Fix test failure in --ps-protocol mode.
mysql-test/t/mysqltest_ps.test:
  Add test case for new --enable_prepare_warning mysqltest command.