(struct File_spec): New member, errnum.

(struct File_spec): New member, errnum.
(recheck): Record the new value of errno in f->errnum.  Don't
output an error message unless the new value of errno differs from
the old one.  Output a message if previously-inaccessible file
becomes accessible.
(tail_forever): Always recheck files whose fd is negative.  If the
file cannot be fstat'ed, record the errno value in f[i].errnum.
(tail_file): If the file cannot be open, record the errno value in
f->errnum.  If it can be opened, initialize f->errnum to zero.  If
it's a non-regular non-fifo file, initialize f->errnum to -1.