Handle /var/tmp mounts when it is a symlink

In Yocto systems /var/tmp is a symlink to /var/volatile/tmp
because in its implementation of read-only rootfs /var is read-only
so /var/volatile is mounted as a tmpfs
and a subset of the paths point into it.

This would result in flatpak emitting mount arguments of
`--symlink ../var/volatile/tmp /var/tmp --bind /var/volatile/tmp /var/volatile/tmp`
which fails because flatpak has already added `--dir /var/tmp`
and the call to symlink fails with EEXIST.

This is fixed by blacklisting /var/tmp from symlink exports
in the same way /tmp is, so the bind is emitted as
`--bind /var/tmp /var/tmp`, which results in /var/volatile/tmp
being mounted into /var/tmp.