[stunnel-users] hex key support for psk

I ran into an issue with PSK reading the key as ascii instead of hex. I had a gnutls-serv and gnutls-cli set up with a hex key, and I switched the server for one wrapped using stunnel, but using the same key in psk.txt was failing. I eventually got it working by converting the hex characters to binary and placing that in the contents of the file specified by PSKsecrets (psk.txt), but this won't work for certain scenarios. For example, what if the key contains ascii newline characters?