Filtering `/dev/urandom` for alphanumeric characters resulted in loss of
input entropy to base64. Fixing this using a procedure with these steps:
* Read 32 bytes from `/dev/urandom` (`dd`)
* Base64-encode (`base64`)
* Strip newlines (`tr -d`)
* URL-Escape (`tr`)
* Append a final newline (`echo`)
This output should be equivalent to output generated using Python and
OpenSSL variants mentioned in the changed document file.
Newlines are stripped as `base64` wraps its output and the option to
disable this (`-w 0`) is not available in all implementations.
Fixes: #1511
Simon Hollenbach