daemon: send stderr of service programs to the syslog

daemon: send stderr of service programs to the syslog

If git-daemon is run with --detach or --inetd, then stderr is explicitly
redirected to /dev/null. But notice that the service programs were spawned
via execl_git_cmd(), in particular, the stderr channel is inherited from
the daemon. This means that errors that the programs wrote to stderr (for
example, via die()), went to /dev/null.

This patch arranges that the daemon does not merely exec the service
program, but forks it and monitors stderr of the child; it writes the
errors that it produces to the daemons log via logerror().

A consequence is that the daemon process remains in memory for the full
duration of the service program, but this cannot be avoided.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>