sysroot: Add ostree_sysroot_write_deployments_with_options()

More sophisticated users of libostree like rpm-ostree need control over things
like the system repository. Previously we introduced a "no cleanup" flag to
`ostree_sysroot_simple_write_deployment()`, but that's a high level API that
does filtering on its own.

Since rpm-ostree needs more control, let's expose the bare essentials of the
"sysroot commit" operation with an extensible options structure, where one of
the options is whether or not to do post-transaction repository operations.

Closes: #745
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -473,6 +473,7 @@ ostree_sysroot_deployment_set_kargs
 ostree_sysroot_deployment_set_mutable
 ostree_sysroot_deployment_unlock
 ostree_sysroot_write_deployments
+ostree_sysroot_write_deployments_with_options
 ostree_sysroot_write_origin_file
 ostree_sysroot_deploy_tree
 ostree_sysroot_get_merge_deployment