Overhaul of 'ar' support:

• use "AR_GNU" as the format name instead of AR_SVR4 (it's what everyone is going to call it anyway)
• Simplify numeric parsing to unsigned (none of the numeric values should ever be negative); don't run off end of numeric fields.
• Finish parsing the common header fields before the next I/O request (which might dump the contents)
• Be smarter about format guessing and trimming filenames.
• Most of the magic values are only used in one place, so just inline them.
• Many more comments.
• Be smarter about handling damaged entries; return something reasonable.
• Call it a "filename table" instead of a "string table"
• Update tests.

Enable selection of 'ar', 'arbsd', and 'argnu' formats by name
(this allows bsdtar to create ar format archives).

The 'ar' writer still needs some work; it should reject
entries that aren't regular files and should probably also
strip leading paths from filenames.