Update Doxyfile

- Make @brief command as an implicit command
- Let doxygen evaluate #ifdef __LINUX__ and #ifdef __FreeBSD__
Some doxygen for flowgrind.h
Delete unused defines
Delete unused base_port variable in struct _opt
More doxygen comments for flowgrind.h
Delete unused members in struct _flow_endpoint
Define flow_endpoint in common.h
No need to define mtu_hints globally
Change struct name _flow to _cflow

Currently, both flowgrind.h as well as daemon.h define a struct _flow.
For the moment it works since the naming scope is different, but maybe
this will change in further. Nevertheless it breaks cscope and
doxygen...
Change GPL Boilerplate

* Say explicit that we are GPL v2 only
* Apply tips from https://www.gnu.org/licenses/gpl-howto.en.html
Update GPL License

At some places the identation was broken. Also some strange special
characters were found. Update GPL License with fresh copy from
http://www.gnu.org/licenses/gpl-2.0.txt.
Parse -U not in 'parse_cmdline' but in 'parse_flow_option'
Delete unnecessary includes
flowgrind's enums do have an underscore in front
Move structs and enums in front of global variables
Some cosmetic changes in guess_topology
Fix doxygen warning
Update Doxyfile
A bit more doxygen comments
Rename sysctl define