sysusers: gracefully handle existing group in /etc/gshadow

sysusers: gracefully handle existing group in /etc/gshadow

systemd-sysusers currently skips entries for groups that already exists.
However, if a group exists only in /etc/gshadow and not in /etc/group,
systemd-sysusers panics:

~# groupdel sgx
~# systemd-sysusers; echo $?
Creating group 'sgx' with GID 106.
0
~# systemd-sysusers; echo $?
0
~# sed -i '/sgx/d' /etc/group
~# systemd-sysusers; echo $?
Creating group 'sgx' with GID 106.
/etc/gshadow: Group "sgx" already exists.
1

To fix this, let's mirror the logic from the user path
(write_temporary_shadow()) where we simply drop the user from the "todo"
queue if it already exists in /etc/shadow instead of throwing an error.