macros: provide %_systemdgeneratordir and %_systemdusergeneratordir (#3672)

macros: provide %_systemdgeneratordir and %_systemdusergeneratordir (#3672)

... as requested in
https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/DJ7HDNRM5JGBSA4HL3UWW5ZGLQDJ6Y7M/.
Adding the macro makes it marginally easier to create generators
for outside projects.

I opted for "generatordir" and "usergeneratordir" to match
%unitdir and %userunitdir. OTOH, "_systemd" prefix makes it obvious
that this is related to systemd. "%_generatordir" would be to generic
of a name.