tail: use consistent diagnostics with and without inotify

tail: use consistent diagnostics with and without inotify

* src/tail.c: With inotify, when a file is initially absent,
we fstat(-1) for that file spec, thus recording an errnum of EBADF,
which caused the "has become accessible" diagnostic to be issued,
when the file first appears.  Instead we avoid the fstat(-1) and
thus emit the more natural and consistent "has appeared" diagnostic.
* tests/tail-2/retry.sh: Use the new diagnostic which also causes
this test to pass on systems without inotify.