Main problem: mysql 5.1 cannot read binlogs from 4.1.
Subproblem 1: There is a mistake in sql_ex_info::init. The read_str()
function updates its first argument to point to the next character to
read. However, it is applied only to a copy of the buffer pointer, so the
real buffer pointer is not updated.
Fix 1: do not take a copy of the buffer pointer. The copy was needed
because sql_ex_info::init does not use the const attribute on some of its
arguments. So we add the const attribute, too.
Subproblem 2: The first BINLOG statement is asserted to be a
FORMAT_DESCRIPTION_LOG_EVENT, but 4.1 binlogs begin with START_EVENT_V3.
Fix 2: allow START_EVENT_V3 too.


mysql-test/suite/binlog/std_data/binlog_old_version_4_1.000001:
  New BitKeeper file ``mysql-test/suite/binlog/std_data/binlog_old_version_4_1.000001''
mysql-test/suite/binlog/r/binlog_old_versions.result:
  Updated result file.
mysql-test/suite/binlog/t/binlog_old_versions.test:
  Added a test reading an old 4.1 binlog.
sql/log_event.cc:
  1. Added const keyword at the following places:
   - input buffer for pretty_print_str
   - input buffer for write_str
   - input buffer, end pointer, and return value from sql_ex_info::init
  2. Fixed the bug by not taking a copy of buf before calling read_str in
  sql_ex_info::init().
sql/log_event.h:
  Added const keyword to fields of the sql_ex_info struct.
  Added const keyword to arguments and return value of sql_ex_info::init
sql/sql_binlog.cc:
  The first BINLOG statement must describe the format for future BINLOG
  statements. Otherwise, we do not know how to read the BINLOG statement.
  Problem: only FORMAT_DESCRIPTION_EVENT is currently allowed as the first
  event. Binlogs from 4.1 begin with a START_EVENT_V3, which serves the
  same purpose.
  Fix: We now allow the first BINLOG statement to be a START_EVENT_V3, as
  well as a FORMAT_DESCRIPTION_EVENT.