user experience improvements (hexin / b64in)

[vanrein]

A few ideas for improving the humanity of the tool:

• skip the wait when data is already being presented (as in copy/paste)
• try to get better looks; perhaps suppress echo and do it explicitly
• when suppressing echo, we can actually provide stylish output:
  • limit lines to 16 well-formatted hex characters, plus ASCII output, like hd
  • space forward lines with half-way addresses to their normal column
• print a space character after the prompt
• print an explicit ^D (or a mere newline) when the input is terminated

The same would apply to b64in, of course. Except that its addresses are a bit weird, including partials.

[vanrein]

And:

• Print the hexin prompt only under isatty(0) to allow input redirection
• Print the hexout prompt only under isatty(1) to allow output redirection

[vanrein]

Implemented in 6afa851 are:

• Print the hexin prompt only under isatty(0) to allow input redirection
• Print the hexout prompt only under isatty(1) to allow output redirection

The user can pipe from/to tee to suppress the prompt for interactive uses. Without prompt, the waiting time could also be removed from hexin.