Upgrade get_version xmlrpc call to return a struct

This commit changes the xmlrpc get_version function to return
additional information (API version, OS name, OS release) instead of just
the flowgrind version string.

Unfortunately this breaks compatibility with earlier versions of flowgrind.
Add/Refine some source code comments
Introduce struct _daemon for info about a flowgrind daemon

Instead of storing unique_server URLs with the flow and
additionally for the xmlrpc client, lets move this info
to a struct _daemon, and reference such an instance from
each flow (endpoints).
Display OS information in the summary line

This commit extends the _daemons struct for API information,
and prints OS info in the summary line. For example:

\#   0 S: localhost (Linux 3.11.0-rc2aha+), random seed: 3789404117, sbuf = 16384/0, rbuf = 87380/0 (real/req), SMSS = 65468, Path MTU = 65520, Interface MTU = 65536 (unknown), flow duration = 10.000s/10.000s (real/req), through = 11674.676753/0.000000Mbit/s (out/in), request blocks = 1781420/0 (out/in)
\#   0 D: localhost (Linux 3.11.0-rc2aha+), random seed: 3789404117, sbuf = 16384/0, rbuf = 87380/0 (real/req), SMSS = 21888, Path MTU = 65520, Interface MTU = 65536 (unknown), through = 0.000000/11674.658181Mbit/s (out/in), request blocks = 0/1781412 (out/in), IAT = 0.002/0.006/0.398 (min/avg/max)
flowgrind: Remove most OS specific ifdefs from the controller
flowgrindd: Remove most OS specific ifdefs from the daemon

Introduces a tcp_info abstraction, so that controller and daemon
know about the same values, regardless on which OS they are compiled.
It should now be possible to use a Linux flowgrind controller with
a FreeBSD daemon and vice versa.
Fix test_adress and server_url logic
flowgrind: remove ifdef on ca state, and document tcp_info limitations on FreeBSD
Merge remote-tracking branch 'origin/master' into interop

Conflicts fixed manually:
	src/common.h
	src/daemon.c
Readd comment which was accidently dropped during merge
Merge pull request #7 from flowgrind/interop

Interop (freebsd <-> linux measurements)