Mosquitto library ported to Genode

[Alexander Weidinger]

Hello,

we successfully ported the mosquitto *library* to Genode (we used
version 15.05 and 16.08 for testing), which may be interesting for some
on the mailing list - it is a library, implementing the MQTT protocol.

https://github.com/argos-research/genode/pull/12

Every option is enabled apart from 'WITH_SRV' which would enable SRV
lookup support.
We didn't test the SSL support, but it is linked and compiled into the
library.

Additionally a simple publisher application is implemented (mpct), which
publishes to a given server a hello world message with an increasing
iterator and makes use of the callback functionality.

One point which took a huge amount of time to find, was that
'lwip_tcpip_init()' function leads to freezes for us
(https://github.com/argos-research/genode/pull/12/files#diff-48dce339109d626747f79ba982b2f8ebR70).
On real hardware (pandaboard / raspberry pi) it freezes before even
receiving an ip address and on the pbxa9 qemu instance it froze after
publishing a few messages, leading to the error message 'Assertion
"tcp_write: pbufs on queue => at least one queue non-empty"'.
To work around this, by simply removing it, everything seems to work and
we are not exactly sure why, maybe someone can explain it to us?

Regards,
Alexander

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