fix sporadic segfault when scanning for hosts
fix race condition while scanning for hosts
Implement dedicated mutex for UI initiated calls
avoid deadlock if the scan is interrupted
enable threading for host scanning in Curses UI
Add an INSTANT_USER_MSG in case of ipv6 probe cancelled
Remove probable fixed old MINGW hack (disable scan threads on MINGW)
Merge pull request #580 from koeppea/fix-579

fix sporadic segfault when scanning for hosts