Move freeze() into shared/

Library code should not call freeze(), this is something that should
only be done by "application code", so moving it into shared/ is appropriate.

The fallback to call _exit() is dropped: let's trust that the infinite loop
is infinite.

src/shared/exec-util.c

@@ -448,6 +448,29 @@ ExecCommandFlags exec_command_flags_from_string(const char *s) {
                 return 1 << idx;
 }
 
+_noreturn_ void freeze(void) {
+        log_close();
+
+        /* Make sure nobody waits for us on a socket anymore */
+        (void) close_all_fds(NULL, 0);
+
+        sync();
+
+        /* Let's not freeze right away, but keep reaping zombies. */
+        for (;;) {
+                int r;
+                siginfo_t si = {};
+
+                r = waitid(P_ALL, 0, &si, WEXITED);
+                if (r < 0 && errno != EINTR)
+                        break;
+        }
+
+        /* waitid() failed with an unexpected error, things are really borked. Freeze now! */
+        for (;;)
+                pause();
+}
+
 int fexecve_or_execve(int executable_fd, const char *executable, char *const argv[], char *const envp[]) {
 #if ENABLE_FEXECVE
         execveat(executable_fd, "", argv, envp, AT_EMPTY_PATH);