beyond unsigned long.
BUG#44779: binlog.binlog_max_extension may be causing failure on 
next test in PB
            
NOTE1: this is the backport to next-mr.
NOTE2: already includes patch for BUG#44779.
            
Binlog file extensions would turn into negative numbers once the
variable used to hold the value reached maximum for signed
long. Consequently, incrementing value to the next (negative) number
would lead to .000000 extension, causing the server to fail.
                        
This patch addresses this issue by not allowing negative extensions
and by returning an error on find_uniq_filename, when the limit is
reached. Additionally, warnings are printed to the error log when the
limit is approaching. FLUSH LOGS will also report warnings to the
user, if the extension number has reached the limit. The limit has been
set to 0x7FFFFFFF as the maximum.

mysql-test/suite/binlog/t/binlog_max_extension.test:
  Test case added that checks the maximum available number for
  binlog extensions.
sql/log.cc:
  Changes to find_uniq_filename and test_if_number.
sql/log.h:
  Added macros with values for MAX_LOG_UNIQUE_FN_EXT and
  LOG_WARN_UNIQUE_FN_EXT_LEFT, as suggested in review.