bin/base64: add line wrapping
bin/base64: add line wrapping

Add line wrapping to the `base64' command line tool to mimic the
behavior of GNU Coreutils `base64'. Encoded lines are wrapped to 76
columns by default, overridable by the user on the command line.

Use vectored writes (using `writev(2)') for faster printing of the
output.

aklomp/base64,aklomp/base64,aklomp/base64