man/systemctl: document enable on masked units
sd-event: be more careful when enabling/disabling signals

When a child event is disabled (in order to be freed) and there is no
SIGCHLD signal event, sd_event_source_set_enabled will disable SIGCHLD
even if there are other child events.

Also remove some unneeded signalfd updates.

https://bugs.freedesktop.org/show_bug.cgi?id=84659

Based-on-a-patch-by: Hristo Venev <mustrumr97@gmail.com>
sd-event: also update signal mask when disconnecting sources
update TODO