fd-util: add wildcard_fd_is_valid() helper and use it tree-wide

fd-util: add wildcard_fd_is_valid() helper and use it tree-wide

Many *at()-style helpers accept a directory fd that may be either a
regular, valid fd (>= 0) or one of the special AT_FDCWD/XAT_FDROOT
wildcard values, and open-code that check in their assertions. Add a
wildcard_fd_is_valid() helper for it and use it tree-wide.