automount: fix handling of default dependencies for automount units

First After=local-fs-pre.target wasn't described in the man page although it's
part of the default dependencies automatically set by pid1.

Secondly, Before=local-fs.target was only set if the automount unit was
generated from the fstab-generator because the dep was explicitly
generated. It was also not documented as a default dependency.

Fix it by managing the dep from pid1 instead.