On Debian 10 (buster) at least, while the Synapse systemd service unit
was referring to Goofys as "matrix-goofys" without a ".service" suffix,
systemd was ignoring the goofys dependency, starting Synapse before
Goofys. All other dependant units which work are using the ".service"
suffix.

This generally leads to the mount path goofys using having been
populated by Synapse before Goofys starts, causing it to fail due to the
mount target not being empty.

The fix seems to be to ensure that the Synapse service unit refers to
Goofys as "matrix-goofys.service".

This change causes the following two lines in
"/etc/systemd/system/matrix-synapse.service":

    Requires=matrix-goofys
    After=matrix-goofys

To be changed to:

    Requires=matrix-goofys.service
    After=matrix-goofys.service