unbound: Enable systemd support and install systemd units

Install systemd socket and service unit.

`--enable-systemd` is needed, as otherwise [the service unit times
out][1].

Also, as we do not store the trust anchor in `/etc/unbound`, RUNDIR in
Unbound’s terms, add `/var/unbound` to the unit’s read/write path.

This does not have an effect yet, as mxtools installs
`/etc/systemd/system/unbound.service`.

For socket activation `use-systemd: yes` has to be set in the
configuration file.

[1]: https://github.com/NLnetLabs/unbound/issues/56

unbound.be0

@@ -56,6 +56,7 @@ SRCURL[0]="https://www.unbound.net/downloads/unbound-${PKGVERSION}.tar.gz"
 mee_configure() {
     bee_configure \
         --disable-static \
+        --enable-systemd \
         --with-pidfile="/run/unbound.pid" \
         --with-rootkey-file="/var/unbound/root.key"
 }
@@ -73,6 +74,8 @@ mee_build() {
 
 mee_install_post() {
     start_cmd mv ${D}/etc/unbound/unbound.conf{,.sample}
+    start_cmd install -m 644 -D -t ${D}/lib/systemd/system ${B}/contrib/unbound.{service,socket}
+    start_cmd sed -i '/^ReadWritePaths=/ s,$, /var/unbound,' ${B}/contrib/unbound.service
 
     start_cmd install -o unbound -m 755 -d ${D}/var/unbound
 }