hostapd: backport support for sending debug messages to the syslog

It wasn't possible to read hostapd wpa_printf messages unless running
hostapd manually. It was because hostapd was printing them using vprintf
and not directly to the syslog.

We were trying to workaround this problem by redirecting STDIN_FILENO
and STDOUT_FILENO but it was working only for the initialization phase.
As soon as hostapd did os_daemonize our solution stopped working.

Please note despite the subject this change doesn't affect debug level
messages only but just everything printed by hostapd with wpa_printf
including MSG_ERROR-s. This makes it even more important as reading
error messages can be quite useful for debugging.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>