otpcli.c: read from standard input with a filename of "-"

otpcli.c: read from standard input with a filename of "-"

It may be convenient to read the secret from standard input in order
to more easily pass it in without otherwise revealing it or having
it be saved in a file.

Allow a filename of "-" to mean read from standard input.

Do, however, refuse to read a binary secret from standard input if
standard input is a tty.