charon-nm: Use better default directory for D-Bus policy file

Also makes it configurable via configure script.  Depending on `$datadir` is
not ideal as package maintainers might set that to a custom value.  Depending
on `$datarootdir` might have been better, the default if pkg-config fails is
now based on that.

References #3339.

src/charon-nm/Makefile.am

@@ -23,7 +23,6 @@ charon_nm_LDADD = \
 	$(top_builddir)/src/libcharon/libcharon.la \
 	-lm $(PTHREADLIB) $(ATOMICLIB) $(DLLIB) ${nm_LIBS}
 
-dbusservicedir = $(datadir)/dbus-1/system.d
-dbusservice_DATA = nm-strongswan-service.conf
+dbuspolicy_DATA = nm-strongswan-service.conf
 
-EXTRA_DIST = $(dbusservice_DATA)
+EXTRA_DIST = $(dbuspolicy_DATA)