Generate alphanumerical random strings.
Prior this change curl used to create random hex strings. This was
mostly okay, but having alphanumerical random strings is better: The
strings have more entropy in the same space.
The MIME multipart boundary used to be mere 64-bits of randomness due
to being 16 hex chars. With these changes the boundary is 22
alphanumerical chars, or little over 130 bits of randomness.
Closes#11838
