Currently, NOTIFY_READY from daemon-util.h conflicts with NOTIFY_READY
from NotifyState from service.h so let's rename the constants to avoid
the conflict.
Let's make use of SD_EVENT_SIGNAL_PROCMASK so that we don't have to mask
the signals manually. Let's use sd_event_set_exit_signal() instead of
rolling our own SIGTERM/SIGINT handling. Let's use "floating" event
sources instead of keeping references on our own.
Let's also debug log if we can't enable watchdog handling.
LLVM 13 introduced `-Wunused-but-set-variable` diagnostic flag, which
trips over some intentionally set-but-not-used variables or variables
attached to cleanup handlers with side effects (`_cleanup_umask_`,
`_cleanup_(notify_on_cleanup)`, `_cleanup_(restore_sigsetp)`, etc.):
```
../src/basic/process-util.c:1257:46: error: variable 'saved_ssp' set but not used [-Werror,-Wunused-but-set-variable]
_cleanup_(restore_sigsetp) sigset_t *saved_ssp = NULL;
^
1 error generated.
```
This is mostly cosmetic, but let's reorder the destructors so that
we do the final sd_notify() call before we run the destructor for
the manager object.
Daan De Meyer
Lennart Poettering
Frantisek Sumsal
Yu Watanabe
Zbigniew Jędrzejewski-Szmek