systemd/systemd-services: manually issue chvt on start of phosh
libseat 0.9.0 no longer performs an automatic VT switch when taking control of a seat to allow sessions to be started in the background. As consequence, one must manually switch if one wants a background session to be immediately activated.
Issue a chvt on ExecStartPost, when the service is deemed running.
Upstream: https://gitlab.gnome.org/World/Phosh/phosh/-/merge_requests/1566
/cc @craftyguy