Add functions to pad the buffer with random strings.

Adds the following commands:

GPGPadRandom: Inserts a random string at the start and end of the
buffer.

GPGRePad: Checks for an existing padding and if found, replaces it.

If the file has the extension '.rpad', then GPGRePad is called
automatically when saving the buffer.
Make GPGReplaceRandom work with folds.