bug#18186: cat.c needs fixing since 1998

[Paul Eggert]

James Simmons, President & CEO wrote:
cat file1.txt file2.txt >>output.txt
file1.txt contains a single line containing IP addresses.
file2.txt contains a single line with more IP addresses.

output.txt SHOULD contain a single line

If the input contains two lines, the output should too.  Sorry, I don't 
see a bug here.