tail: prefer < 0 to == -1

tail: prefer < 0 to == -1

* src/tail.c (valid_file_spec, recheck, writers_are_dead)
(tail_forever, check_fspec, tail_forever_inotify, tail_file)
(parse_options, main): Be a bit more systematic about checking
for sign, rather than for exact equality or inequality,
when the sign is enough.  Makes the code a bit clearer
now that -2 sometimes means success.