Add a new -Dllvm-fuzz=true option that can be used to build against
libFuzzer and update the oss-fuzz script to work outside of the
oss-fuzz build environment.
The fuzz targets are intended to be fast and only target systemd
code, so they don't need to call out to any dependencies. They also
shouldn't depend on shared libraries outside of libc, so we disable
every dependency when compiling against oss-fuzz. This also
simplifies the upstream build environment significantly.
The fuzzers will be used by oss-fuzz to automatically and
continuously fuzz systemd.
This commit includes the build tooling necessary to build fuzz
targets, and a fuzzer for the DNS packet parser.
Jonathan Rudenberg