Skip .DS_Store files in server side utils

Skip .DS_Store files in server side utils

The macOS Finder application creates .DS_Store files in directories
when opened,  which creates problems for serverside utilities which
expect all files to be PostgreSQL specific files.  Skip these files
when encountered in pg_checksums, pg_rewind and pg_basebackup.

This was extracted from a larger patchset for skipping hidden files
and system files, where the concencus was to just skip these. Since
this is equally likely to happen in every version, backpatch to all
supported versions.

Reported-by: Mark Guertin 
Reviewed-by: Michael Paquier 
Reviewed-by: Tobias Bussmann 
Discussion: https://postgr.es/m/E258CE50-AB0E-455D-8AAD-BB4FE8F882FB@gmail.com
Backpatch-through: v12