debian: Optimize build process (compile in build stage not in binary stage)

doclib: Fix some senf.css / doxygen.css interactions
Packets: Implement ConcreatePacket::parser
Packets: Make the PacketTypeMixin work with (simple) dynamically sized parsers

Packets: Renamed PacketParserMixin::registry_key_t to key_t
Packets: Add PacketParserMixin::lookup helper

admin: Make build.sh do a parallel build and use 'nice'
admin: Make build.sh import an optional DOXYGEN envvar
senfscons: Allow setting arbitrary simple build environment variables from the command line

HowTos/NewPacket: Started the packet type section
doclib: Implemented the '\autotoc' command
Packets: Add SENF_PACKET_REGISTRY_REGISTER macro

doclib: Find optimal graph rotation for large dot graphs
doclib: Break long pathnames in dot graphs
doclib: Use new DOT_GRAPH_MAX_NODES parameter in Doxyfile.global
doclib: Break lines in (textual) class inheritance lists

Examples: Autogenerate Mainpage.dox
HowTos/NewPacket: Start new howto