icinga2.service used `-e ${ICINGA2_ERROR_LOG}`, but this is documented
as having no effect without `-d`. Furthermore, icinga2 under systemd
unconditionally logged everything to the system log (but without setting
the log level etc), which contradicted the documentation. (Issue #6339)
Stop icinga2 on systemd from logging to stdout - and hence the system log -
once it has finished starting up. Just like when you start icinga2 from a
terminal using `-d`. And just like -d, we stop logging fatal errors to
stderr, and instead write to the log file passed with `-e`.
As per docs, mainlog (icinga2.log) is already enabled by default. And
pre-startup messages including config errors will still appear in the
system log.
This uses a new option --close-stdio, which has the same effect on logging as
--daemonize, but does not fork or call setsid().
For this purpose, I moved setsid() up and into Daemonize().
Consequence of that last point: if anyone is weird enough to specify a TTY
device file as the fatal error log (-e option), that will become icinga's
controlling terminal, which you generally don't want as a daemon. This
makes it consistent with the existing behaviour for icinga mainlog. For
this reason you're supposed to use O_NOCTTY in Linux daemons. But I wasn't
sure where icinga would want to put the ugly `#ifdef _WIN32 ... #else ...`.