Meson: Support pdns-auth and ixfrdist service files

Meson: Support pdns-auth and ixfrdist service files

Service files are treated like a config.h.in file. This adds support for a common base of
service file configuration options containing basic systemd feature checks.

Then, each of pdns-auth and ixfrdist have their own "general" and "instance" service files
that are generated from a common service file. This is why things like @Description@,
@ConfigName@ and @Config@ are made generic so that each version of the service file can
use it own string.