it seems like some linux newbies might try to manage the cert & key by
themselves or thirdparty software (such as caddy), and complain about
hysteria start failure due to permission denied.
and it is actually really pretty hard to teach them how to edit systemd
units, or place their certs & key files into a suitable directory and
set proper permission.
which this commit, we can easily tell them just re-run the install
script with the following command:
HYSTERIA_USER=root ./install_server.sh -f
this script can also detect the correct user from the installed systemd
units, so it still works properly even if the user forgets to specify
HYSTERIA_USER= in the subsequent runs.
