Short term support for wifi-only downloads

[Barry Warsaw]

For the upcoming RTM, we want to improve wifi-only downloads, but in a very conservative way. Post-RTM we can consider doing something more elaborate, based on any specifications we receive from design.

For now:

 * s-i will call the allowGSMDownload() passing in the appropriate flag before the group download is started.
 * s-i will attempt to cancel any in-progress download when it exits due to timeout (watch for object path being missing)

 * u-d-m will pause when network changes to prevent download (i.e. wifi-only switching to gsm)
 * u-d-m will cancel downloads if they are paused for a long time (it must also clean up any partially downloaded files)

 * no changes currently for system-settings

[Barry Warsaw (barry)]

<gatox> i think here "u-d-m will cancel downloads if they are paused
        for a long time (it must also clean up any partially downloaded
        files)" it's missing what mandel said about checking if the process is
        still running or not before cleaning... mandel is that right?

[Barry Warsaw (barry)]

Point of order: When the auto_download setting is set to 0, meaning manual downloads, we'll take that as allowing GSM downloads under the assumption that if downloads are manually invoked, the user knows what they are doing w.r.t. GSM connectivity. If that turns out not to be the case, then we'll need additional settings to differentiate, but we'll let design initiate that change.