xlog: erase eof marker when reopening existing file for writing

When reopening an existing xlog file (as in case of vylog), we do not
erase the eof marker immediately. Instead we reposition file offset
to (file_size - sizeof eof_marker), assuming the eof marker will be
overwritten on the first write.

However, it isn't enough if we want to reuse this function for reopening
WAL files, because when scanning the WAL directory we close a file if we
read eof marker and never reopen it again, see recover_remaining_wals().
So to avoid skipping rows written to a once closed WAL, we have to erase
the eof marker when reopening an xlog file. Let's do it with truncate().