osutil: recognize SNAPD_HOME_REMOTE_FS=1

[zyga]

When SNAPD_HOME_REMOTE_FS is set to a "true" value such as "1", then snapd assumes that /home is a remote file system mounted in ways more complex than static entry in /etc/fstab.

This allows us to support cifs-mounted home handled by pam_mount, without having to restart snapd to get correct sandbox permissions (and to re-compile every profile again) during the login process.

This is related to ongoing work to support cifs-mounted home, handled with pam_mount, as smoothly as we can: https://forum.snapcraft.io/t/using-snapd-with-cifs-smb3-mounted-home-directory/39616

[ernestl]

This concept requires a spec (as suggested by Samuele). Therefore, not part of 2.63 content anymore.

[zyga]

I've created an internal spec SD169 - Host Configuration File