log_checkpoint(), log_make_checkpoint_at(): Remove the parameter
write_always. It seems that the primary purpose of this parameter
was to ensure in the function recv_reset_logs() that both checkpoint
header pages will be overwritten, when the function is called from
the never-enabled function recv_recovery_from_archive_start().

create_log_files(): Merge recv_reset_logs() to its only caller.

Debug instrumentation: Prefer to flush the redo log, instead of
triggering a redo log checkpoint.

page_header_set_field(): Disable a debug assertion that will
always fail due to MDEV-19344, now that we no longer initiate
a redo log checkpoint before an injected crash.

In recv_reset_logs() there used to be two calls to
log_make_checkpoint_at(). The apparent purpose of this was
to ensure that both InnoDB redo log checkpoint header pages
will be initialized or overwritten.
The second call was removed (without any explanation) in MySQL 5.6.3:
mysql/mysql-server@4ca37968da54ddc6b3b6628f41428ddba1c79bb8

In MySQL 5.6.8 WL#6494, starting with
mysql/mysql-server@00a0ba8ad92569fcf08212b3b8cf046dc8a0ce10
the function recv_reset_logs() was not only invoked during
InnoDB data file initialization, but also during a regular
startup when the redo log is being resized.

mysql/mysql-server@45e91679832219e2593c77185342f11f85232b58
in MySQL 5.7.2 removed the UNIV_LOG_ARCHIVE code, but still
did not remove the parameter write_always.