tree-wide: introduce new safe_fork() helper and port everything over

This adds a new safe_fork() wrapper around fork() and makes use of it
everywhere. The new wrapper does a couple of things we previously did
manually and separately in a safer, more correct and automatic way:

1. Optionally resets signal handlers/mask in the child

2. Sets a name on all processes we fork off right after forking off (and
   the patch assigns useful names for all processes we fork off now,
   following a systematic naming scheme: always enclosed in () – in order
   to indicate that these are not proper, exec()ed processes, but only
   forked off children, and if the process is long-running with only our
   own code, without execve()'ing something else, it gets am "sd-" prefix.)

3. Optionally closes all file descriptors in the child

4. Optionally sets a PR_SET_DEATHSIG to SIGTERM in the child, in a safe
   way so that the parent dying before this happens being handled
   safely.

5. Optionally reopens the logs

6. Optionally connects stdin/stdout/stderr to /dev/null

7. Debug logs about the forked off processes.

src/basic/process-util.h

@@ -142,3 +142,17 @@ int ioprio_parse_priority(const char *s, int *ret);
 pid_t getpid_cached(void);
 
 int must_be_root(void);
+
+typedef enum ForkFlags {
+        FORK_RESET_SIGNALS = 1U << 0,
+        FORK_CLOSE_ALL_FDS = 1U << 1,
+        FORK_DEATHSIG      = 1U << 2,
+        FORK_NULL_STDIO    = 1U << 3,
+        FORK_REOPEN_LOG    = 1U << 4,
+} ForkFlags;
+
+int safe_fork_full(const char *name, const int except_fds[], size_t n_except_fds, ForkFlags flags, pid_t *ret_pid);
+
+static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
+        return safe_fork_full(name, NULL, 0, flags, ret_pid);
+}