Adding 'RuntimeDirectory' is needed when pidfile path is set to
subdirectory under /run.
Adding ConfigurationDirectory may help in some non-standard setups.
Also add more descriptions about used rules to avoid user confusion
about they meaning and purpose.

The real purpose of this service is to make it work with
https://systemd.io/PORTABLE_SERVICES/ which are incompatible with
chroot workarounds from original unbound.service.
The service content is identical to unbound.service with exception
for chroot related rules which were modified as needed.

State directory will be created under /var/lib/unbound and will be
useful for writing various files managed at runtime like trust
anchors updates there instead of in ConfigureDirectory which could
be made read-only next. For this chroot needs to be disabled.

CAP_CHOWN is needed for changing onwership of pidfile before
dropping privileges and truncate pidfile on exit.

CAP_IPC_LOCK controls whether a process can lock pages into physical
memory (for instance to prevent passwords or private keys from
being swapped to disk), e.g. mmap() with the MAP_LOCKED flag or
shmctl() with the SHM_LOCK command, neither of which seem to be
used by unbound.

Pidfiles aren't needed while running unbound through systemd.
The PID of the unbound daemon can still be obtained with:
'systemctl show --property MainPID --value unbound'.
While disabling pidfiles we can also drop CAP_CHOWN and writable
/run directory.