sysupdate: Split the update verb into two parts internally

An ‘acquire’ (download) part, and an ‘install’ (apply) part.

Following commits will expose these as separate verbs and D-Bus methods,
but this commit is the one which rearranges the internals.

If doing an ‘install’, a mirror version of the ‘acquire’ has to happen
first to make sure the transfer’s internal state is correct.

‘Acquire’ can require an internet connection, but ‘install’ will always
work with `--offline` specified.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/34814

src/sysupdate/sysupdate-partition.h

@@ -36,6 +36,7 @@ typedef struct PartitionInfo {
         }
 
 void partition_info_destroy(PartitionInfo *p);
+int partition_info_copy(PartitionInfo *dest, const PartitionInfo *src);
 
 int read_partition_info(struct fdisk_context *c, struct fdisk_table *t, size_t i, PartitionInfo *ret);