Similarly to the crash fixed by #2967, which happens if sudo failed,
a crash also happens when sudo even fails to start. The reason for
the crash is also similar: nil dereference of screen.Screen caused by
the fact that we do not restore temporarily disabled screen.
To reproduce this crash, set the `sucmd` option to some non-existing
command, e.g. `aaa`, and try to save a file with root privileges.
On modern Linux systems, it can take 30 seconds for
the data to actually hit the disk (check
/proc/sys/vm/dirty_expire_centisecs).
If the computer crashes in those 30 seconds, the user
may end up with an empty file as seen here:
https://github.com/neovim/neovim/issues/9888
This is why editors like vim and nano call
the fsync syscall after they wrote the file.
This syscall is available as file.Sync() in Go.
Running strace against micro shows that fsync is
called as expected:
$ strace -f -p $(pgrep micro) -e fsync
strace: Process 3284344 attached with 9 threads
[pid 3284351] fsync(8) = 0
Also, we now catch errors returned from w.Flush().
