Add signal handler class
Add signal handler class

This signal handler class abstracts some of the code previously
used in other places. It also uninstalls its handler when Reset()
is called or the class is destructed, thereby restoring the
previous behaviour.

Reviewed-by: iustinp


onponomarev/ganeti,ganeti/ganeti,grnet/snf-ganeti,dimara/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,bitemyapp/ganeti,leshchevds/ganeti,ganeti/ganeti,andir/ganeti,grnet/snf-ganeti,apyrgio/ganeti,yiannist/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti,leshchevds/ganeti,andir/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,yiannist/ganeti,mbakke/ganeti,dimara/ganeti,andir/ganeti