pfx: replace socket_id member with a pointer to the rtr_socket

Replace the socket_id member in a pfx_record, with a socket member from
type rtr_socket *. The socket member references the rtr_socket that
retrieved the pfx_record.

The previously stored socket_id also contained the memory address of the
rtr_socket but was from type uintptr_t. Converting it to type rtr_socket *
enables library users to access informations about the rtr_socket that
retrieved a particular record.
examples: fix wrong tr_socket initializations

The char * members of the tr_sockets were initialized with a char[].
This doesn't work, a pointer is normally smaller than eg a hostname
string.
rtr_mgr: remove array bound check in rtr_mgr_status_to_str

Trust the user that he uses the function correctly and only pass valid
rtr_mgr_status values instead of random integers.

The previous array bound check also was wrong. The sizeof operations
returned the total bytes allocated of the array and not the members of
the array.
rtrt: remove array bound check in rtr_state_to_str

Trust the user that he uses the function correctly and only pass valid
rtr_socket_state values instead of random integers.

The previous array bound check also was wrong. The sizeof operations
returned the total bytes allocated of the array and not the members of
the array.
tmp
debian packaging files

work in progress :)
cmake: add UseMultiArch cmake module

Source: https://raw.github.com/OPM/opm-upscaling/master/cmake/Modules/UseMultiArch.cmake

The cmake module fixes wrong multiarch installation paths when cmake is
called from debhelper
cmake: build documentation by default

if doxygen is present, build the documentation in the all target
cmake: install documentation to share/doc/rtrlib/
cmake: move rtrclient install statement to tools/CMakeLists.txt

All cmake statement related to files in tools should be in
tools/CmakeLists.txt
This commit moves the cmake install install statement for the rtrclient
tools to tools/CmakeLists.txt.
fixup! debian packaging files