Instead of treating any lxcfs mount as an indicator that we're running
in a container, check for one of the mounts actually used by LXC
containers.
For reference, here's a list of mounts I am seeing in an LXC container:
```
$ grep lxcfs /proc/mounts
lxcfs /proc/cpuinfo fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/diskstats fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/loadavg fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/meminfo fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/stat fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/swaps fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /proc/uptime fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
lxcfs /sys/devices/system/cpu/online fuse.lxcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
```
Fixes#8444
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
2cf6e12790