mesg: avoid 'ttyname failed: Success' message

mesg: avoid 'ttyname failed: Success' message

The ttyname(3) can fail to access /dev/ path, and that will cause function
to fail without setting errno value with result of rather confusing error
message.  Lets start setting stdin permission via /proc when this happens as
a go-around, with hope kernel following symlink does not fail.  Ok, noted,
that hopes of symlink follow working are pretty slim.

Based on patch from Sami Kerola <kerolasa@iki.fi>.

Reference: https://github.com/lxc/lxd/issues/1724
Signed-off-by: Karel Zak <kzak@redhat.com>