info: write debugging output to file if GST_DEBUG_FILE environment variable is set

info: write debugging output to file if GST_DEBUG_FILE environment variable is set

This changes behaviour slightly in that we no longer output things
via g_printerr(), so any non-standard glib printerr handlers are no
longer called when GST_DEBUG is enabled. However, this seems not
really desirable in most cases anyway, and the GLib docs also say
that libraries should not use g_printerr() for logging.

Other stderr output (e.g. warnings, or application messages) will
of course not be captured in the log file this way.

GST_DEBUG_FILE=- will redirect debug output to stdout.