BibSched: fix conflict between OS signals and I/O

• Connected USR2 signal to SIG_IGN. This should solve any conflict raising when pinging a task with USR2 signal for discovering if it's still alive, while the task is performing I/O operation (e.g. downloading an URL). (See http://www.python.org/doc/2.3.5/lib/module-signal.html, the fourth dot).

• This solves notably problem with full-text-indexing remote files.

(cherry-picked from d7f4373a235c808caccabb068587d7a8afc23422)