Support mounting /sysroot (and /boot) read-only

We want to support extending the read-only state to cover `/sysroot`
and `/boot`, since conceptually all of the data there should only
be written via libostree.  Or at least for `/boot` should *mostly*
just be written by ostree.

This change needs to be opt-in though to avoid breaking anyone.

Add a `sysroot/readonly` key to the repository config which instructs
`ostree-remount.service` to ensure `/sysroot` is read-only.  This
requires a bit of a dance because `/sysroot` is actually the same
filesystem as `/`; so we make `/etc` a writable bind mount in this case.

We also need to handle `/var` in the "OSTree default" case of a bind
mount; the systemd generator now looks at the writability state of
`/sysroot` and uses that to determine whether it should have the
`var.mount` unit happen before or after `ostree-remount.service.`

Also add an API to instruct the libostree shared library
that the caller has created a new mount namespace.  This way
we can freely remount read-write.

This approach extends upon in a much better way previous work
we did to support remounting `/boot` read-write.

Closes: https://github.com/ostreedev/ostree/issues/1265

src/libostree/ostree-impl-system-generator.c

@@ -28,6 +28,7 @@
 #ifdef HAVE_LIBMOUNT
 #include <libmount.h>
 #endif
+#include <sys/statvfs.h>
 #include <stdbool.h>
 #include "otutil.h"
 
@@ -189,8 +190,6 @@ _ostree_impl_system_generator (const char *ostree_cmdline,
                                "[Unit]\n"
                                "Documentation=man:ostree(1)\n"
                                "ConditionKernelCommandLine=!systemd.volatile\n"
-                               /* We need /sysroot mounted writable first */
-                               "After=ostree-remount.service\n"
                                "Before=local-fs.target\n"
                                "\n"
                                "[Mount]\n"