supervise-daemon: allow merging stderr and stdout

[WhyNotHugo]

Implement a --merge-stderr flag for supervise-daemon, along with a merge_stderr for openrc-run. This merges stderr into stdout, allowing a single process (via --output_logger/output_logger) to handle all output from a service.

Implements: #1019

[navi-desu]

i think i'd more in favor of redirecting stderr to stdout if only output_log is set (and maybe don't if output_log="foo" error_log=)

after releasing 0.64 i've promised to revamp logging in openrc (so we no longer /dev/null them by default either) so i'll take a look at this then

[WhyNotHugo]

after releasing 0.64 i've promised to revamp logging in openrc (so we no longer /dev/null them by default either) so i'll take a look at this then

I'd love to be able to configure logging globally. E.g.: a default for all services which don't override it. An expected usage would be something like:

output_logger="logger -t $SERVICE_NAME -p daemon.info"

[WhyNotHugo]

i think i'd more in favor of redirecting stderr to stdout if only output_log is set (and maybe don't if output_log="foo" error_log=)

That sounds very sensible; I can't imagine that "I want to log stdout and discard stderr" is a common use case.