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/async.c

@@ -61,29 +61,18 @@ finish:
 }
 
 int asynchronous_sync(void) {
-        pid_t pid;
+        int r;
 
         /* This forks off an invocation of fork() as a child process, in order to initiate synchronization to
          * disk. Note that we implement this as helper process rather than thread as we don't want the sync() to hang our
          * original process ever, and a thread would do that as the process can't exit with threads hanging in blocking
          * syscalls. */
 
-        log_debug("Spawning new process for sync");
-
-        pid = fork();
-        if (pid < 0)
-                return -errno;
-
-        if (pid == 0) {
+        r = safe_fork("(sd-sync)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS, NULL);
+        if (r < 0)
+                return r;
+        if (r == 0) {
                 /* Child process */
-
-                (void) rename_process("(sd-sync)");
-
-                (void) reset_all_signal_handlers();
-                (void) reset_signal_mask();
-
-                (void) close_all_fds(NULL, 0);
-
                 (void) sync();
                 _exit(EXIT_SUCCESS);
         }