Switch to internal API and add fallback user for fstatat()

- Add `fstatat_with_fallback_user` function to handle UIDs in `my_fstatat.c`
- Add `#include <stdbool.h>` and `bool retried` to `proc_parse.c`
- Add `retry_o_path` goto and `if (!retried)` check to `proc_parse.c`
- Use `fstatat_with_fallback_user

[criu/my_fstatat.c]
- Add `fstatat_with_fallback_user` function
- Set the UID based on the `CURRENT_TASK_USER` environment variable
- Rollback to the original UID after the call to `fstatat`
[criu/proc_parse.c]
- Add `#include <stdbool.h>`
- Add `bool retried`
- Add `retry_o_path` goto
- Add `if (!retried)` check
- Add `flags = O_PATH`
[criu/files-reg.c]
- Include `my_fstatat.c`
- Use `fstatat_with_fallback_user` instead of `fstatat`