Fix signal handling.

Fix signal handling.

Using signal() to set the disposition of a signal is always a bad idea
as POSIX does not specify whether a system call which was interrupted
should be restarted or not.

For interactive sessions, the Linux behaviour is to automatically
restart slow system calls, specifically read(2) in case nothing had
been read before the interrupt arrived. This is rather unfortunate
as adu calls fgets(3) (hence read(2)) in an endless loop to read the
user input. Therefore automatically restarted read() calls result
in interactive sessions that can not be terminated easily, as was
noticed by Sebastian Stark.

This patch makes the signal initialization code call sigaction()
instead of signal() to set up the handlers. This buys us well-defined
semantics across all operating systems, namely to *not* restart slow
system calls. As a side effect of this change, interactive sessions
can now be terminated by sending SIGINT (e.g., by pressing CTRL+C).