bin/base64: modernize the demo program

[aklomp]

The library has long contained a demo program, bin/base64.c, that gives a working demonstration of how the library could be used. The program intentionally mimics the behavior of GNU Coreutils base64.

This program is pretty old and is not as polished or well-written as it could be. Improve it in the following ways:

• Use stdbool types for returning success/fail from functions.
• Add more comments to explain what is going on.
• Add proper command line options parsing, and add a help function.
• Default to printing encoded output in 76-char columns, like GNU Coreutils base64 does.
• Print all error messages to stderr.