Mention that systemd::unit and systemd::unit_options automatically
includes and notifies systemd::daemon_reload, so users don't have to
do that themselves.

Have the error messages that unitfile.erb can raise include reference
to the resource (type and title) causing the error, so users can find
where they made the error.

When setting ensure = absent to systemd::unit_options, it doesn't
make sense for users to have to specify the options parameter.  But
it was a mandatory parameter, so they had to.  Fix that.

In systemd::unit_options, you can prefix an option name with a minus
sign (Unicode 002D "hyphen-minus") to "reset" a list option to the
empty list, so the new values don't just append to the existing list
of values.  That makes sense when overriding/supplementing options in
earlier unit files, but systemd::unit installs the "main" unit file,
the one that is read first, so list options *always* have the empty
list as their value when the file is read.

We here modify the unitfile.erb template file to see if it is called
from systemd::unit or systemd::unit_options, and if the former, it
raises an error if it encounters an option name starting with a minus
sign.  And of course also remove the paragraph describing that beha-
viour in the 'options' parameter documentation.

This adds a new definition, systemd::unit, for managing unit files
in /etc/systemd/system.  The content of the unit file can be given
as a hash of hashes (like in the systemd::unit_options definition),
as a string, or as a path or puppet: URL to a file to copy.

Since we use the same ERB template file for formatting the hash of
hashes into an ini-style file in both systemd::unit_options and
systemd::unit, we have renamed the template file to "unitfile.erb",
to be more neutral.  The comment at the end mentioning from where
the file is managed, now takes the Puppet definition as a parameter,
set by the definition.

This adds the semi-internal class 'systemd::vars' containing various
paths and other constants for use in the rest of the module, so we
won't need to repeat ourselves.  Currently only contains a single
variable, $etcdir, pointing to /etc/systemd/system.

The default case in the case switch for the $ensure parameter was
incorrectly using the *string* "default" instead of the keyword
``default''.  This meant that you did not get an error message if
you specified an illegal value for that parameter (unless it happened
to be exactly the string "default").

This adds the class 'systemd::daemon_reload', which can be used
to tell systemd to re-read its service configuration files.

This will contain a Puppet module for helping managing the systemd(1)
init system and systemd service configuration files.