Add engine private key password support

Some OpenSSL engines (selectable via tls_engine option) may require a
password to make use of private keys created with them in the first place.

The TPM engine for example, will require a password to access the underlying
TPM's Storage Root Key (SRK), which is the root key of a hierarchy of keys
associated with a TPM; it is generated within a TPM and is a non-migratable
key. Each owned TPM contains a SRK, generated by the TPM at the request
of the Owner. [1]

By default, the engine will prompt the user to introduce the SRK password
before any private keys created with the engine can be used. This could
be inconvenient when running on an unattended system.

Here's where the new tls_engine_kpass_sha option comes in handy. The user
can specify a SHA1 hash of its engine private key password via command
line or config file and it will be passed on to the engine directly.

This commit adds support for both clients (libmosquitto) and broker.

[1] https://goo.gl/qQoXBY

Signed-off-by: Nicolás Pernas Maradei <nicopernas@gmail.com>

lib/cpp/mosquittopp.cpp

@@ -376,4 +376,9 @@ int mosquittopp::tls_keyform_set(const char *keyform)
    return mosquitto_tls_keyform_set(m_mosq, keyform);
 }
 
+int mosquittopp::tls_engine_kpass_sha_set(const char *kpass_sha)
+{
+   return mosquitto_tls_engine_kpass_sha_set(m_mosq, kpass_sha);
+}
+
 }