od: fix data padding

[mknos]

• Odd length input was not producing the correct output in all cases; I found this with od -l
• Make a general padding function to avoid duplication in the formatting functions
• Single-byte formatting functions, such as octal1() and char1(), don't require any padding

%wc -c a.c
197 a.c
%od -l a.c | tail -n 2   # gnu reference
0000300  1933534308          10
0000305
%perl od -l a.c | tail -n 2   # before patch; no value of 10 in output
00000300 1933534308 
00000305

[briandfoy]

changes: fix odd cases in output padding