tail: don’t output more lines than requested

tail: don’t output more lines than requested

* src/tail.c (file_lines): Fix an unlikely bug where ‘tail -n N’
could output more than N lines if standard input is a largish
regular file with large initial offset that starts with (say) N-1
lines after the initial offset, but grows to N+1 lines between the
fstat and read calls.  In this case ‘tail -n N’ now outputs N-1
lines, not N+1; that is, it pretends the file grew after ‘tail’
read it.  That is better than outputting more than N lines.