Fix unblocking of signals in interactive mode

Fix unblocking of signals in interactive mode

In interactive mode (-I2), those signals that were blocked at startup
will remain blocked for the whole period of strace execution.

* strace.c (empty_set): Rename to start_set.  All users changed.
(init): Initialize start_set and blocked_set to the set of currently
blocked signals.
(startup_attach, trace): Use SIG_SETMASK instead of SIG_BLOCK.
* NEWS: Mention this change.