platform: use also statvfs() to check for udevd

Check whether or not there is a running udevd by using statvfs() on
"/sys" and use access() as a fallback. This is in line with what is
done by systemd [1] and helps in case NM is not really running in a
container but has been confined by a MAC so it does not have full
access to sysfs (access() returns EACCES).

[1] https://github.com/systemd/systemd/blob/v246/src/basic/stat-util.c#L132

!639
(cherry picked from commit 9c8275be)