mount: honor --timeout-idle-sec=SEC option (#41010)

mount: honor --timeout-idle-sec=SEC option (#41010)

When using systemd-mount to create a transient .mount/.automount file
for removable storage, the option to specify the idle timeout on the
commandline using **--timeout-idle-sec=SEC** is not reflected in the
generated .automount file. Instead, the idle timeout is always set to 1
second.

arg_timeout_idle_set was never set to true when passing the argument, so
arg_timeout_idle was always set to 1s.

Fixes #41007.

Co-authored-by: patrick <patrick@localhost>