tail: prefer intmax_t to uintmax_t

tail: prefer intmax_t to uintmax_t

Signed types let us debug better, by using -fsanitize=undefined.
* doc/local.mk (doc/constants.texi):
Adjust change from macro to enum.
* src/tail.c (COPY_TO_EOF, COPY_A_BUFFER)
(DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS):
Now enum constants, not macros.
(COPY_TO_EOF, COPY_A_BUFFER): Now negative, not positive.
(count_t): New typedef.  Use it instead of uintmax_t.
(COUNT_MAX): New macro; use it instead of UINTMAX_MAX.
(struct File_spec, max_n_unchanged_stats_between_opens)
(dump_remainder, file_lines, pipe_lines, pipe_bytes)
(start_bytes, start_lines, tail_forever, check_fspec)
(tail_forever_inotify, tail_bytes, tail_lines, tail, tail_file)
(parse_obsolete_option, parse_options, main):
Prefer count_t to uintmax_t.