Fix systemd-import services

Fix systemd-import services

On debian, systemd complains about missing /bin/awk because it
actually is located at /usr/bin/awk. It is not a good idea to
hardcode binary paths because different linux distros use different
paths. According to systemd's man page it is absolutely safe to
miss paths for binaries located at standard locations (/bin,
/sbin, /usr/bin, ...).

Further, replace this more or less complicated awk command by
grep.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Issue #8510