Use _exit() instead of exit() in child process

exit() should not be called from within a child process, since it would
call functions registered with atexit(), and flush stdio streams. Use
_exit() instead.

app/src/unix/command.c

@@ -14,7 +14,7 @@ pid_t cmd_execute(const char *path, const char *const argv[]) {
     if (pid == 0) {
         execvp(path, (char *const *)argv);
         perror("exec");
-        exit(1);
+        _exit(1);
     }
     return pid;
 }