init: improve handling of signals racing with each other

Before this change, a request to reboot could be "overwritten" by e.g.
SIGHUP.

function                                             old     new   delta
init_main                                            709     793     +84
packed_usage                                       33273   33337     +64
run_actions                                          109     117      +8
stop_handler                                          87      88      +1
check_delayed_sigs                                   340     335      -5
run                                                  214     198     -16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/2 up/down: 157/-21)           Total: 136 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>