fuzzenv: Introduce utility for temporary runtime directories

In fuzzing it's important to not depend on external configuration and, if fuzzers are run in parallel, to not interact between them. Fuzzer frameworks such as Honggfuzz automatically mount a temporary filesystem (tmpfs) on certain directories (sometimes configurable). The utility contained in this change can be invoked early in a daemon's "main" function to write a dynamic "smb.conf" file pointing to paths in the temporary directory.

The list of directories is not complete. Only those needed to run "smbd" and "winbindd" are defined, in addition to two shared directories.