maint: prefer same-inode.h

maint: prefer same-inode.h

This does not change behavior on POSIX platforms; it’s mostly to
make it clearer when we’re looking for file identity.
* src/cat.c (main):
* src/copy.c (struct dir_list, is_ancestor, copy_internal):
* src/tail.c (struct File_spec, record_open_fd, recheck)
(tail_forever_inotify, tail_file):
* src/test.c (binary_operator):
Use psame_inode, PSAME_INODE, or SAME_INODE instead of comparing
device and inode numbers by hand.