With automount the target mount drive appears twice in /proc/self/mountinfo.
379 27 0:70 / /mnt/rclone rw,relatime shared:433 - autofs systemd-1 rw,fd=57,...
566 379 0:90 / /mnt/rclone rw,nosuid,nodev,relatime shared:488 - fuse.rclone remote: rw,...
Before this fix we only looked for the mount once in
/proc/self/mountinfo. It finds the automount line and since this
doesn't have fs type rclone it concludes the mount isn't ready yet.
This patch makes rclone look through all the mounts and if any of them
have fs type rclone it concludes the mount is ready.
See: https://forum.rclone.org/t/systemd-mount-works-but-automount-does-not/42287/
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
Current way of checking whether mountpoint has been already mounted (directory
list) can result in race if rclone runs under Automount (classic or systemd).
This patch adopts Linux ProcFS for the check. Note that mountpoint is considered
empty if it's tagged as "mounted" by autofs. Also ProcFS is used to check whether
rclone mount was successful (ie. tagged by a string containing "rclone").
On macOS/BSD where ProcFS is unavailable the old method is still used.
This patch also moves a few utility functions unchanged to utils.go:
CheckOverlap, CheckAllowings, SetVolumeName.
