Fix incorrect @return comment in method documentation.

Issue: PLCR-521
Add SecureRandom::uniform(); provides uniformally distributed random numbers.

Issue: PLCR-521
Implement async-safe version of mktemp().

This uses our new SecureRandom API, and (non-recursively) will attempt
permutes the generated result to try every possible value (matching the
implementation of mkstemp()).

The previous approach of simply generating a random file name could
result in non-termination in the case that all possible file
combinations were already on-disk.

Issue: PLCR-521