fix: yet another fd leak
chg: tempstr() now return an empty string by default
fix: check openssl error codes to avoid segfault

EVP_Cipher() was crashing when the cipher initialization failed
(for instance, when the cipher suite is not supported by openssl).
chg: add a reference from set-proto-enabled doc to proto-stats

For it may not be obvious that the corresponding getter for set-proto-enabled
is actually proto-stats, not an inexistant get-proto-enabled.
chg: dev: Make sure that the socket is closed when the Guile thread terminate.