Bug#827814: accept multiple watch lines as one group

[Osamu Aoki]

Hi,

This is just a design note:

Currently, one watch line (after concatenating physical lines if there
is a tailing \) corresponds to one file download.

Let's introduce "group" and "priority" to opts to use multiple lines to
describe one download.

Let's make the "watch" file to consist of multiple download groups which
downloads multiple tarballs of components and their pgp signature files

As for this download "group" which may be made of multiple lines in the
watch file.  opts is used as follows:
 * group=first-last    : 1 line only  as a download group (default)
 * group=first-middle  : first  line  of a download group
 * group=middle-middle : middle lines of a download group
 * group=middle-last   : last   line  of a download group
 
As for the "priority" in opts, this is user provided weighting figure
for each line to chose the best download resource.  Smaller the better.

Regards,

Osamu