execute: check whether the specified fd is a tty before chowning/chmoding it (#3457)

execute: check whether the specified fd is a tty before chowning/chmoding  it (#3457)

Let's add an extra safety check before we chmod/chown a TTY to the right user,
as we might end up having connected something to STDIN/STDOUT that is actually
not a TTY, even though this might have been requested, due to permissive
StandardInput= settings or transient service activation with fds passed in.

Fixes:

https://bugs.freedesktop.org/show_bug.cgi?id=85255