maria_read_log used to always print a warning message at startup
to say it is unsafe if ALTER TABLE was used. Now it prints it only
if the log does show the problem (=ALTER TABLE or CREATE SELECT, which
both disable logging of REDO_INSERT*).
For that, when ha_maria::external_lock() disables transactionality
it writes a LOGREC_INCOMPLETE_LOG to the log, which "maria_read_log -a"
picks up to write a warning.
REPAIR TABLE also disables those REDO_INSERT* but as maria_read_log
executes LOGREC_REDO_REPAIR no warning is needed.


storage/maria/ha_maria.cc:
  as we now log a record when disabling transactionility, we need the
  TRN to be set up first
storage/maria/ma_blockrec.c:
  comment
storage/maria/ma_loghandler.c:
  new type of log record
storage/maria/ma_loghandler.h:
  new type of log record
storage/maria/ma_recovery.c:
  * maria_apply_log() now returns a count of warnings. What currently
  produces warnings is:
  - skipping applying UNDOs though there are some (=> inconsistent table)
  - replaying log (in maria_read_log) though the log contains some
  ALTER TABLE or CREATE SELECT (log misses REDO_INSERT* for those
  and is so incomplete).
  Count of warnings affects the final message of maria_read_log and
  recovery (though in recovery none of the two conditions above should
  happen).
  * maria_read_log used to always print a warning message at startup
  to say it is unsafe if ALTER TABLE was used. Now it prints it only
  if the log does show the problem, i.e. ALTER TABLE or CREATE SELECT
  was used (both disable logging of REDO_INSERT* as those records are
  not needed for recovery; those missing records in turn make
  recreation-from-scratch, via maria_read_log, impossible). For that,
  when ha_maria::external_lock() disables transactionality,
  _ma_tmp_disable_logging_for_table() writes a LOGREC_INCOMPLETE_LOG to
  the log, which maria_apply_log() picks up to write a warning.
storage/maria/ma_recovery.h:
  maria_apply_log() returns a count of warnings
storage/maria/maria_def.h:
  _ma_tmp_disable_logging_for_table() grows so becomes a function
storage/maria/maria_read_log.c:
  maria_apply_log can now return a count of warnings, to temper the
  "SUCCESS" message printed in the end by maria_read_log.
  Advise users to make a backup first.