sysupdate: Ensure Context is offline by default

Since being online is a privileged operation, it’s better to fail-safe
and make the `Context` offline by default unless explicitly initialised
(e.g. with `context_from_cmdline()` or an explicit initialisation of
`.offline = false`).

There are a few instances where `.offline = true` is set explicitly.
I’ve left those in place, even though they’re now redundant, as
documentation.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
(cherry picked from commit 97183383af03279c43d9c270af5832d36d527d01)

src/sysupdate/sysupdate.c

@@ -94,6 +94,7 @@ COMMAND(
                 .sync = true,                                     \
                 .instances_max = UINT64_MAX,                      \
                 .verify = -1,                                     \
+                .offline = true,                                  \
                 .cleanup = -1,                                    \
                 .installdb_fd = -EBADF,                           \
                 .target_identifier.class = _TARGET_CLASS_INVALID, \