daemon/os: check for download-only and dry-run conflict

Minor follow-up to previous commit. It seems a bit confusing to allow
specifying `download-only` and `dry-run`. The former already includes
all the steps in the latter but goes further, as documented. Let's check
for this combination.

Closes: #1155
Approved by: cgwalters

src/daemon/org.projectatomic.rpmostree1.xml

@@ -286,7 +286,7 @@
             Update rpmmd repo metadata cache and ostree refspec. Do not
             perform any deployments. This is like "dry-run" except that
             the latter does not download and import packages. Not valid
-            if "cache-only" is specified.
+            if "cache-only" or "dry-run" is specified.
     -->
     <method name="UpdateDeployment">
       <arg type="a{sv}" name="modifiers" direction="in"/>

src/daemon/rpmostreed-os.c

@@ -661,6 +661,9 @@ start_deployment_txn (GDBusMethodInvocation  *invocation,
   if (vardict_lookup_bool (&options_dict, "cache-only", FALSE) &&
       vardict_lookup_bool (&options_dict, "download-only", FALSE))
     return glnx_null_throw (error, "Can't specify cache-only and download-only");
+  if (vardict_lookup_bool (&options_dict, "dry-run", FALSE) &&
+      vardict_lookup_bool (&options_dict, "download-only", FALSE))
+    return glnx_null_throw (error, "Can't specify dry-run and download-only");
   if (override_replace_pkgs)
     return glnx_null_throw (error, "Non-local replacement overrides not implemented yet");