This tries to get rid of most manual sigprocmask() changes, in favour
of:
1. The SD_EVENT_SIGNAL_PROCMASK flag to sd_event_add_signal()
2. The sd_event_set_signal_exit() call for handling SIGTERM/SIGINT
3. Move masking of SIGWINCH into ptyfwd, out of nspawn/vmspawn/run
And while we are at it get rid of a bunch of event source fields whose
lifetime is bound to the sd_event object they belong to anyway, and make
use of the "floating" event source feature of sd-event instead.
Lennart Poettering
d7a6bb9891