ostree-sysroot: make simple_write_deployment smarter

This is a follow-up to https://github.com/ostreedev/ostree/pull/1097.
We make simple_write_deployment smart enough so that it can be used for
rpm-ostree's purposes. This is mostly an upstreaming of logic that
already existed there.

Notably we correctly append NOT_DEFAULT deployments *after* the booted
deployment and we now support RETAIN_PENDING and RETAIN_ROLLBACK flags
to have more granularity on deployment pruning.

Expose these new flags on the CLI using new options (as well as expose
the previously existing NOT_DEFAULT flag as --not-as-default).

I couldn't add tests for --retain-pending because the merge deployment
is always the topmost one. Though I did check that it worked in a VM.

Closes: #1110
Approved by: cgwalters

man/ostree-admin-deploy.xml

@@ -89,6 +89,30 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--retain-pending</option></term>
+
+                <listitem><para>
+                    Do not delete pending deployments.
+                </para></listitem>
+            </varlistentry>
+
+            <varlistentry>
+                <term><option>--retain-rollback</option></term>
+
+                <listitem><para>
+                    Do not delete rollback deployments.
+                </para></listitem>
+            </varlistentry>
+
+            <varlistentry>
+                <term><option>--not-as-default</option></term>
+
+                <listitem><para>
+                    Append rather than prepend new deployment.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--karg-proc-cmdline</option></term>