more: improve error messaging when input file is directory

more: improve error messaging when input file is directory

Before this change the error message about directory was not informative,
and a little bit out of place.

    $ touch empty ; ./more ./empty /
    ::::::::::::::
    ./empty
    ::::::::::::::
    magic failed: cannot read fd 4 (Is a directory)
    ::::::::::::::
    /
    ::::::::::::::

After the change messaging is reverted back to what it used to be.

    $ ./more ./empty /
    ::::::::::::::
    ./empty
    ::::::::::::::

    *** /: directory ***

And while on it lets remove repeated fstat() call.

Fixes: 09070e1a658e70ec203150e4fa5f486b32771858
Signed-off-by: Sami Kerola <kerolasa@iki.fi>