sd-device: fix enumeration of devices without subsystem

sd-device: fix enumeration of devices without subsystem

Prior to commit c32eb440bab953a0169cd207dfef5cad16dfb340, libudev's
function udev_enumerate_scan_devices() had behaved differently. If
parent match was added with udev_enumerate_add_match_parent(),
udev_enumerate_scan_devices() did not return error if some child devices
had no subsystem symlink in sysfs. An example of such devices is USB
endpoints /sys/bus/usb/devices/*/ep_*. If there was a parent match
against USB device, old implementation of udev_enumerate_scan_devices()
did not treat ep_* device directories without subsystem symlink as error
and just ignored them, but new implementation returns -ENOENT (also
ignoring these devices) though correctly enumerates all other matching
devices.

To compare, you could look at 96df036fe3d25525a44f5efdb2fc8560e82e6cfd,
in src/libudev/libudev-enumerate.c, function parent_add_child():

    if (!match_subsystem(enumerate, udev_device_get_subsystem(dev)))
            goto nomatch;

udev_device_get_subsystem() was returning NULL, match_subsystem() was
returning false, and USB endpoint device was ignored.

New parent_add_child() from src/libsystemd/sd-device/device-enumerator.c
checks return value of sd_device_get_subsystem() and fails if subsystem
was not found. Absence of subsystem symlink should not be really treated
as error because all enumerations of children of USB devices will fail
with -ENOENT. This new behavior also breaks system-config-printer.

So restore old behavior and treat absence of subsystem symlink as no
match.