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

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.