pidfd-util: open an internal pidfd if none is passed in pidfd_check_pidfs()

pidfd-util: open an internal pidfd if none is passed in pidfd_check_pidfs()

I'd like to introduce a libsystemd helper for acquiring pidfd
inode id, which however means the fd passed to pidfd_check_pidfs()
can no longer be trusted. Let's add back the logic of allocating
a genuine pidfd allocated internally, which was remove in
5dc9d5b4eacbe32f58ad6ca18d70931ab89ea409.