log: skip reading the kernel command line if the process is invoked by a script

CLI tools may be used in a script. E.g., a script for monitoring a
service may use `systemctl`. Previously, if the kernel command line has
e.g. systemd.log-level=debug, then systemctl in the script produces
debugging logs when the script is invoked by a .service unit, but does
not when the script is running in a terminal. Then,
https://github.com/systemd/systemd/pull/18281#discussion_r561697482,
> I expect users to be (negatively) surprised.

In the previous commit, $SYSTEMD_EXEC_PID= is introduced. Then, we can
now detect whether a command is directly invoked by systemd or through
a script. Let's skip reading the kernel command line when a command is
invoked through a script.