shuf: speed-up when limited output is requested

A user noted that the following command was slower than they
expected:

   busybox shuf -i "1500000000-$(date +%s)" -n 5

At time of writing the range contains 128 million values.  On my
system this takes 7.7s whereas 'shuf' from coreutils takes a
handful of milliseconds.

Optimise BusyBox 'shuf' for cases where -n is specified by stopping
shuffling once the required number of lines have been processed.
On my system the time for the example is reduced to 0.4s.

function                                             old     new   delta
shuf_main                                            520     540     +20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 20/0)               Total: 20 bytes

v2: Code shrink.  Since outlines <= numlines:
    - the loop in shuffle_lines() only needs to test the value of
      outlines;
    - shuffle_lines() can be called unconditionally.
    Update timing to allow for the 13 million seconds elapsed since v1.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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