core: implement Uphold= dependency type

This is like a really strong version of Wants=, that keeps starting the
specified unit if it is ever found inactive.

This is an alternative to Restart= inside a unit, acknowledging the fact
that whether to keep restarting the unit is sometimes not a property of
the unit itself but the state of the system.

This implements a part of what #4263 requests. i.e. there's no
distinction between "always" and "opportunistic". We just dumbly
implement "always" and become active whenever we see no job queued for
an inactive unit that is supposed to be upheld.

man/systemd.unit.xml

@@ -708,6 +708,24 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>Upholds=</varname></term>
+
+        <listitem><para>Configures dependencies similar to <varname>Wants=</varname>, but as long a this unit
+        is up, all units listed in <varname>Upholds=</varname> are started whenever found to be inactive or
+        failed, and no job is queued for them. While a <varname>Wants=</varname> dependency on another unit
+        has a one-time effect when this units started, a <varname>Upholds=</varname> dependency on it has a
+        continuous effect, constantly restarting the unit if necessary. This is an alternative to the
+        <varname>Restart=</varname> setting of service units, to ensure they are kept running whatever
+        happens.</para>
+
+        <para>When <varname>Upholds=b.service</varname> is used on <filename>a.service</filename>, this
+        dependency will show as <varname>UpheldBy=a.service</varname> in the property listing of
+        <filename>b.service</filename>. The <varname>UpheldBy=</varname> dependency cannot be specified
+        directly.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>Conflicts=</varname></term>