install: beef up preset logic to limit to only enable or only disable, and do all-unit preset operations

The new "systemctl preset-all" command may now be used to put all
installed units back into the enable/disable state the vendor/admin
encoded in preset files.

Also, introduce "systemctl --preset-mode=enable-only" and "systemctl
--preset-mode=disable-only" to only apply the enable or only the disable
operations of a "systemctl preset" or "systemctl preset-all" operation.

"systemctl preset-all" implements this RFE:

https://bugzilla.redhat.com/show_bug.cgi?id=630174

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