Add constant for base 64 encoded filter

[jimhester]

I needed to modify the whitespace to properly align the columns. If you would prefer I do that in a separate commit let me know.

There don't seem to be any current tests for archive_write_add_filter(), so I did not add a test for this change.

[kientzle]

That constant is also used to identify the format when reading an archive. So if we really want to distinguish uu format from base64 format, then we should also update:

https://github.com/libarchive/libarchive/blob/master/libarchive/archive_read_append_filter.c#L80

and

https://github.com/libarchive/libarchive/blob/master/libarchive/archive_write_add_filter_b64encode.c#L101

and we would also want to set the detected format appropriately here:

https://github.com/libarchive/libarchive/blob/master/libarchive/archive_read_support_filter_uu.c#L367

We should certainly add a test for archive_write_add_filter(), we can probably start by copying test_archive_write_add_filter_by_name.c. We should also update test_write_filter_b64encode.c to verify that the correct format code is being reflected in the write pipeline data that can be queried through archive_filter_code() and friends.

[jsonn]

I would just call it B64, similar to UU.