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). This fixes https://bugs.launchpad.net/snappy-hwe-snaps/+bug/1712918
[1] https://github.com/systemd/systemd/blob/v246/src/basic/stat-util.c#L132
Edited by Alfonso Sanchez-Beato