avoid blocking+malloc below signal_handler (bnc#929826)

[bmwiedemann]

the old code would deadlock and block until killed with kill -9

https://bugzilla.opensuse.org/show_bug.cgi?id=929826

see also
http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_03
for a list of signal-safe functions

[bmwiedemann]

note: some segfaults still left

[mlandres]

The only clean solution is leaving the exit handler and exiting ASAP. Appdata trigger, cleanup of temp. repos, directories, etc.. must be performed.

[mlandres]

Should be resolved meanwhile. The exit handler is left ASAP and should not perform any cleanup anymore.