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>

Add same OpenSSL engine support to mosquitto (server side) previously added to
client side only.

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

- Clients can now offload crypto tasks to an external crypto device through
  the OpenSSL ENGINE API.
- The keyfiles can now be treated as PEM or ENGINE keys.
- Two new functions were added to libmosquitto to set up the previously
  mentioned features.
- Both mosquitto_sub and mosquitto_pub include support to turn on the mentioned
  features through command line options.

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

Plus tests.

This is for broker outgoing connack and publish packets only.

Closes #1169. Thanks to Christoph Krey.

Closes #1167. Thanks to Christoph Krey.

This now returns MOSQ_ERR_INVAL as expected.

This now returns MOSQ_ERR_INVAL as expected.

Thanks to momoskitto.

Produce error if this is not the case. Closes #1164, thanks to
momoskitto.

Closes #1144. Thanks to Kris Mattheus.

This occurred when per_listener_settings was set to true.

Closes #1081. Thanks to dwin-wangjt.

Closes #1137. Thanks to Kris Mattheus.