Problem: As, wsrep_provider is dynamic and can accept
a valid provider while server is running. Patch for
#4222 wrongly skips the initialization of binlog plugin
during server start, when wsrep provider is not specified.

In wsrep build, if opt_log_bin is not provided, we still
init binlog with wsrep_emulate_bin_log ON.

Fix: This patch fixes #4222 by adding a check to return
from binlog_savepoint_set without proceeding further if
wsrep_on is OFF/false.

This patch also fixes MDEV#5407 and reverts changes pushed
for MDEV#5386 (no longer valid).