Since bug@20166, which replaced the binlog file name generating to base
on pidfile_name instead of the previous glob_hostname, the binlog file
name suddenly started to be stored solely in the absolute path format,
including a case when --log-bin option meant a relative path.
What's more serious, the path for binlog file can lead unrequestedly 
to pid-file directory so that after any proper fix for this bug
there might be similar to the bug report consequences for one who
upgrades from post-fix-bug@20166-pre-fix-bug@28597 to post-fix-bug@28597.

Fixed with preserving`pidfile_name' (intr.by bug@20166) but stripping
off its directory part. This restores the original logics of storing
the names in compatible with --log-bin option format and with the
requirement for --log-bin ralative path to corresond to the data directory.
Side effects for this fix:

effective fixing bug@27070, refining its test;
ensuring no overrun for buff can happen anymore (Bug#31836 
insufficient space reserved for the suffix of relay log file name);
bug#31837  --remove_file $MYSQLTEST_VARDIR/tmp/bug14157.sql missed
in rpl_temporary.test;
fixes Bug@28603  Invalid log-bin default location;