bpf: untangle usage of common.h

Re-organize common.h a bit so that it's possible to build simple Cilium-related programs (like bpf_sock_term or bpf_probes) without including the whole world. I'd hope that with a bit more progress on this effort, we can also avoid the include in bpf_alignchecker.

Moving many of the map-related structs into their associated header should also help with better code ownership assignment.