checkdirs() was being passed the wrong mount point, resulting in a panic

checkdirs() was being passed the wrong mount point, resulting in a panic
when mounting over an already existing mountpoint and improperly adjusting
the current or root directory for processes when finding a matching ncp
whether they were relative to the old mount or not.

Rewrite and document checkdirs() to fix the problems.

Reported-by: "Vincent Labrecque" <vnc@hush.ai>