Post-pushbuild fix.

- Windows does not have 'socket' system variable.
- Compiler warning in sql/slave.cc

mysql-test/r/variables.result:
  Bug#28234 - global/session scope - documentation vs implementation
  Updated test result.
mysql-test/t/variables.test:
  Bug#28234 - global/session scope - documentation vs implementation
  Removed test for 'socket' variable. Windows doesn't have it.
sql/slave.cc:
  Bug#28234 - global/session scope - documentation vs implementation
  Changed type of constant to avoid a compiler warning.