od: implement -a option

[mknos]

• BSD and GNU versions support -a for printing literal characters, with control character names printed
• GNU version deliberately masks highest (8th) bit per byte, so we follow this
• This is consistent with standards document[1] wording: "named characters from the International Reference Version (IRV) of the ISO/IEC 646:1991 standard. Only the least significant seven bits of each byte shall be used"
• Standards document doesn't mention -a flag, but generally -a is taken to mean the same as "-t a"
• Attempt to document the available options
• When testing this against GNU "od -An -a", whitespace alignment of characters was different but "diff -w" showed no difference for a random test file

1. http://ktiml.mff.cuni.cz/~kucerap/unix/susv4tc2/utilities/od.html

[briandfoy]

changes: add -a option to print control character names