Teach pg_ls_dir_files() to ignore ENOENT failures from stat().

 Teach pg_ls_dir_files() to ignore ENOENT failures from stat().
 
 Buildfarm experience shows that this function can fail with ENOENT
 if some other process unlinks a file between when we read the directory
 entry and when we try to stat() it.  The problem is old but we had
 not noticed it until 085b6b667 added regression test coverage.
 
 To fix, just ignore ENOENT failures.  There is one other case that
 this might hide: a symlink that points to nowhere.  That seems okay
 though, at least better than erroring.
 
 Back-patch to v10 where this function was added, since the regression
 test cases were too.
 
 Discussion: https://postgr.es/m/20200308173103.GC1357@telsasoft.com