tree-wide: make sigprocmask() changes more automatic

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.

src/basic/signal-util.h

@@ -28,7 +28,7 @@ int sigaction_many_internal(const struct sigaction *sa, ...);
 int sigset_add_many_internal(sigset_t *ss, ...);
 #define sigset_add_many(...) sigset_add_many_internal(__VA_ARGS__, -1)
 
-int sigprocmask_many_internal(int how, sigset_t *old, ...);
+int sigprocmask_many_internal(int how, sigset_t *ret_old_mask, ...);
 #define sigprocmask_many(...) sigprocmask_many_internal(__VA_ARGS__, -1)
 
 const char* signal_to_string(int i) _const_;