Improve pg_check_dir code and comments.

Improve pg_check_dir code and comments.

Avoid losing errno if readdir() fails and closedir() works.  Consistently
return 4 rather than 3 if both a lost+found directory and other files are
found, rather than returning one value or the other depending on the
order of the directory listing.  Update comments to match the actual
behavior.

These oversights date to commits 6f03927fce038096f53ca67eeab9adb24938f8a6
and 17f15239325a88581bb4f9cf91d38005f1f52d69.

Marco Nenciarini