core: add IgnoreOnSoftReboot= unit option

core: add IgnoreOnSoftReboot= unit option

As it says on the tin, configures the unit to survive a soft reboot.
Currently all the following options have to be set by hand:

Conflicts=reboot.target kexec.target poweroff.target halt.target
Before=reboot.target kexec.target poweroff.target halt.target
After=sysinit.target basic.target
DefaultDependencies=no
IgnoreOnIsolate=yes

This is not very user friendly. If new default dependencies are added,
or new shutdown/reboot types, they also have to be added manually.

The new option is much simpler, easy to find, and does the right thing
by default.