rtr_mgr: make rtr_mgr_config a private data structure

Previously the rtr_mgr_config definition was publicly available and
parts from it had to be initialized from the user.

The definition moved to rtr_mgr.c and isn't public anymore. This
prevents that library access attributes of the struct rtr_mgr_config
while they are in use from another thread of the library. It was also
confusing for the user that he only had to initialize parts of
the struct rtr_mgr_config.

In consequence the rtr_mgr_init(..) signature was changed. The
rtr_mgr_config parameters was removed instead it now returns
a pointer to an initialized rtr_mgr_config or NULL if an error occured.
rtr_mgr_free(..) deallocates the memory allocated by the rtr_mgr_conf,
not only from the memory of the attributes in the rtr_mgr_conf.
rtr_mgr: add a connection status callback

Add a callback function to the rtr_mgr that allows the library user to
react on connection status changes.

rtr_mgr_init(..) has 2 new parameters:
- A pointer to a function that is called on every connection status
  change.
- A void pointer that is passed to the rtr_mgr_status_fp function. The
  referenced memory area can be used freely by the library user.
rtr_mgr: pass the rtr_socket to the status callback function

The rtr_mgr status callback is now called for every status change of an
rtr_socket and the rtr_socket is passed to the callback function.
The library user knows exactly which server connection changed the
status and can eg update the status of alle server connection as soon as
they changed.
rtr: add function rtr_state_to_str(..)

Add the function rtr_state_to_str(..). The function takes an
rtr_socket_state and returns the state as String.
rtr_mgr: add function rtr_mgr_status_to_str(..)

Add the function rtr_mgr_status_to_str(..). The function takes an
rtr_mgr_status and returns the status as string.
rtrclient: print connection status changes

Use the new rtr_mgr connection status callback to print the status of
the rtr_socket and rtr_mgr whenever a socket changed his connection
state
cmake: remove -pedantic compiler flag
rtr: add missing include pthread.

add missing include pthread.h
packets.c: initialize a variable to supress a clang warning

initialize rtval to suppress a clang warning. The case that rtval was
returned unitialized couldn't happened, because it was ensured in
another function that flag is 0 or 1
doxygen: adapt examples to latest changes
cmake: rework Debug mode compile flags

Many new warnings to fix :)
rtrlib: fix missing static keywords
tests: remove unused functions