This splits out a bunch of functions from fileio.c that have to do with temporary files. Simply to make the header files a bit shorter, and to group things more nicely. No code changes, just some rearranging of source files.
According to https://oss-fuzz.com/fuzzer-stats/by-fuzzer/fuzzer/libFuzzer/job/libfuzzer_asan_systemd, fuzz-network-parser, fuzz-netdev-parser and fuzz-journal-remote produce a lot of unwanted log lines. Let's set the maximum log level to LOG_CRIT as we do in the other fuzzers.