base64,base32: fix 'extra operand' error message

base64,base32: fix 'extra operand' error message

In the following invocation, 'a' is the input file, and 'b' is the extra
operand:

  $ base64 a b

Report 'b' in the error message instead of 'a':

  $ base64 a b
  base64: extra operand 'b'

Discussed in https://lists.gnu.org/r/coreutils/2018-12/msg00008.html .

* src/basenc.c (main): If there is more than one non-option operand,
report the second one (assuming the first is a the input file name).
* tests/misc/base64.pl: Add tests.
* tests/misc/basenc.pl: Adjust expectedc error message in tests.
* NEWS: Mention bugfix.