try to minimize bugzilla queries

This attempts to solve several deficiencies in the script:
1. If the first attachment of a bug is already downloaded, the bug is
   not checked for newly added attachments (or attachments with newly
   fixed mimetype).
2. If neither of the eligible attachment(s) of a bug is the first
   attachment, the bug will be processed (and the attachment downloaded)
   time and again (because the shortcut is only applied for the first
   attachment, see 1).

But it also ensures that if the script is killed, the download is
restarted on the next run.

Change-Id: I7f3d1922825bb314f96ec3b1ee2a0ac47604b018
enable more bugzillas

Change-Id: If77b213deba85d49e1e8d09015fa81aa6df6aa94
put bz URLs into a list to avoid repeated code

Change-Id: Iade61f598e1122aae7667de684a68e8164817327
print bug count in rss query too

Change-Id: I9542771fe3caf77757d81641752ab5463ead4deb