`tac /proc/modules' would print nothing

`tac /proc/modules' would print nothing

(copy_to_temp): Renamed from save_stdin, since
now it copies a general file descriptor, not just stdin.
(tac_nonseekable): Renamed/adapted from tac_stdin.
(tac_file): Get fd via `open' directly rather than via fopen/fileno,
since we never used the stream.  Perform "-" to stdin mapping here
rather than in main.  Determine whether a file is seekable,
by trying to `lseek' to its end, and dispatch to tac_seekable or
tac_nonseekable accordingly.
(main): Rewrite argument handling now that it uses only tac_file.
Reported by Harald Dunkel in http://bugs.debian.org/278604.