For pastealacon, it is used to check for encoding compatibility.

Use stderr and return codes >0 when it makes sense.
Loose return code conventions:
* 1 for generic errors
* 2 for user input errors
* 3 for remote errors (not found, etc.)
* 4 for not implemented

This is useful for testing backends with different configurations
(different sites, being anonymous or not, etc.)
The previous behavior was to chose a random backend instance.

Basic support in pastoob added (there is no way yet to change the
default options).
Backends support the "public/private" requirement.

Unless we are writing in a file or in a pipe

And fix the capability.

Do not use print (as it add a newline), fix the wanted fields, use "get"
instead of "show".

For now it can only get the contents of a paste.