#10 Fix signal handler

[Markus Elfring]

Pthread functions do not belong to the list of async-signal-safe functions.
See section "2.4.3 Signal Actions" from the document "2.4 Signal Concepts".
http://opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_03

I have got doubts about the use of "warning" in this context.

Should the corresponding calls be replaced by safe alternatives in your function "stop_signal"?
http://wifiscanner.svn.sourceforge.net/viewvc/wifiscanner/trunk/daily/src/scanner.c?revision=158&view=markup