ueventd: Add `devname sys_name` to `subsystem`, pulling device names from sysfs

This change introduces a new mode to `subsystem.devname` in `ueventd.rc`
configuration files, which sets the file name to the contents of
`/sys/DEVNAME/name`.

The objective of this change is to help Cuttlefish distinguish between
console devices, which are only different in uevents by initialization
order. Cuttlefish currently relies on `/dev/hvc##` devices which are
created for non-multiport virtio-console devices.

https://cs.android.com/android/platform/superproject/main/+/main:device/google/cuttlefish/shared/config/ueventd.rc;l=18;drc=5204f119d859d3ae5f1a2ee1c6a05ee68d6a28ed

On Cuttlefish we're considering moving to multiport virtio-console
devices ( https://fedoraproject.org/wiki/Features/VirtioSerial ). It
would be possible to rely on device order here as well, but using names
to distinguish devices makes it possible to drop unused devices in the
future, rather than reserving indexes indefinitely.

Multiport virtio-console devices create uevents with DEVNAME=vport#p#
and DEVPATH=.../vport#p#, only exposing the name in a sysfs file.

Bug: 336663898
Test: Attach multiport console, run with `-DLOG_UEVENTS=1`
Test: Introduce ueventd policy using `devname sys_name`
Change-Id: I59632b556db4a47883eab97e90c0e6ca81a9c650