zsh-completion: systemctl: pass only $PREFIX* to list-unit*

zsh-completion: systemctl: pass only $PREFIX* to list-unit*

Using a leading * and $SUFFIX produces misleading results. Let's imagine
that one mistypes nect instead of netc, they will get a rather
misleading completion like:  sys-fs-fuse-connections.mount

Not to mention that the execution time is up by ~1/3.
time systemctl list-unit-files netctl* -> ~12ms
time systemctl list-unit-files *netctl* -> ~17ms

Furthermore more units are matched, leading to greater execution time
of `systemctl show' in _filter_units_by_property

Use only $PREFIX*, removing the leading * and trailing $SUFFIX*.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>