The final conceptual patch addresses the 1st item for the last commit todo.
That includes

a. extending rpl_checksum to demonstrate slave execution stability 
in face of binary and relay-log rotations in the context of master and slave
having arbitrary (possibly different) server's checksum algorithms.

b. prior the test to pass some piece of code was clean up to simplify logics of
   need_checksum() and propagation of the relay-log checksum Alg from one
   master-slave session to the other.

c. binlog_checksum is converted from bool to enum forcing changes in external and internal
    interfaces as well as the master-slave binlog-checksum handshake.

This patch leaves the reliable FD issue out to be addresses in a separate piece of work.
That will implement an idea described on WL#2540 which is about
`we need to distinguish between a checksummed-and-corrupted FD and not checksummed'.

TODO:

1. extend the test base to verify all cover cases of OM -> NS and NM -> OS
2. merge with bug#49741


Fine details of changes per file:



mysql-test/extra/rpl_tests/rpl_log.test

	gave out to wait for  bug#49741 merge to edit myself


mysql-test/include/have_binlog_checksum_off.inc

        a new guard to require by tests which mess with simulation 
	incompatible checksum ways.


mysql-test/suite/rpl/t/rpl_checksum.test
 
	to demonstrate slave execution stability in face of binary and
        relay-log rotations in the context of master and slave having
        arbitrary (possibly different) server's checksum algorithms.


sql/log.cc

	Addressing, binlog-checksum bool-to-enum-conversion, RL-checksum placeholder logics
	simplification, Log_event::need_checksum() correcting and relaxing.

 sql/log.h

        Commenting on the definition of relay_log_checksum_alg as the placeholder
	of  Relay-LoG  checksum alg value.

sql/log_event.cc

       	Addressing, binlog-checksum bool-to-enum-conversion, RL-checksum placeholder logics
	simplification, Log_event::need_checksum() correcting and relaxing.

 sql/rpl_mi.h
sql/rpl_mi.cc

         RL-checksum placeholder logics simplification. The last seen value of
	 FD_m.A (master's FD checksum alg) is stored in relay_log's member right
	 upon receiving the event. That makes unnecessary ealier ramped up
	 last_master_checksum_alg.

sql/mysqld.c
sql/mysql_priv.h
 sql/log_event.h

        Addressing, binlog-checksum bool-to-enum-conversion.


sql/set_var.h

	 Addressing binlog-checksum bool-to-enum-conversion.
	 *Notice* change (private->protected for `value' member) in a base class 
	 which is similar to one done ago to  sys_var_set.


 sql/slave.cc

        Addressing binlog-checksum bool-to-enum-conversion incl master-slave handshake,
	equality of FD_q.A with RL.checksum_alg.
	
	
sql/sql_repl.cc

	Addressing binlog-checksum bool-to-enum-conversion via
	redifing the binlog-checksum sys_var class.