Rework build of fuzzers.

- Do not require any libraries installed, download everything
from upstream git, statically compile (use include, libs and pkg-config
from local directory under tests/fuzz).
Script should work both from OSS-Fuzz and locally.

- Do not require local protobuf (only staticallly compiled, see above).

- Add README.md (TBD, still not finished).

- Fix make dist and distcheck.

- Remove common.[ch] as we can use internal function.
  This makes fuzzers also C++ only (remove CFLAGS from Makefile).

tests/fuzz/crypt2_load_proto_plain_json_fuzz.cc

@@ -33,8 +33,6 @@ extern "C" {
 #include <err.h>
 #include <fcntl.h>
 #include <unistd.h>
-
-#include "common.h"
 }
 
 DEFINE_PROTO_FUZZER(const json_proto::LUKS2_both_headers &headers) {