bouquet: Allow merging of services across network bouquet, fixes #5617
In a mixed network environment (such as DVB-T and DVB-S) it is common
to receive many of the same channels. However, with "auto map to
channels" on a bouquet, you could not merge the channels across the
network bouquets.
Now we create a separate "Tvheadend Network" in the bouquets and allow
"auto map to channels" on it. Any other bouquets that are enabled but
do _not_ have "auto map to channels" enabled will be added to the
enabled "Tvheadend Network".
So, if "DVB-T Network" and "DVB-S Network" in the bouquet tab are both
enabled _and_ their tickbox for "auto map to channels" is deselected,
then their services will be added to the "Tvheadend Network". That can
then be enabled with "merge channels" to produce the merged channels.
We call it "Tvheadend Network" instead of "Tvheaded Virtual Bouquet"
to be consistent with the other auto-generated names.
The new bouquet is lazily updated to ensure the CPU overhead is
minimized. This means that when other bouquets are scanned, we wait
until the system is quiet before updating the new bouquet with
the new channels.
