bt-agent: add option to redirect stdout and stderr to syslog

[corviv]

Add handlers for g_print() and g_printerr() methods to redirect output to syslog.

Add the corresponding option to use it.

[WhyNotHugo]

Can't we just pipe to logger(1)?

[corviv]

Can't we just pipe to logger(1)?

Hi @WhyNotHugo I've added --syslog/-s mainly for Buildroot, but it helps in any minimal image (BusyBox, Alpine, small containers, etc.):

• zero extra deps: no external logger(1) needed → smaller rootfs;
• no extra forks — logs go straight through syslog();
• correct severity (LOG_INFO/LOG_ERR) even if stdout/err are closed;
• optional, default-off.

It also integrates cleanly with systemd-journald because /dev/log is forwarded automatically. Thanks for feedback.

[WhyNotHugo]

busybox includes logger(1).

[corviv]

Correct, busybox can include logger(1), but it’s an optional applet that many size-optimised Buildroot or container configs leave out (CONFIG_LOGGER option). Besides, even when it exists, every log line goes through a separate fork()+exec() + see previous arguments.

So the option mainly removes a conditional dependency and gives tighter resource control, while staying completely optional.