update local_copy_dir rsync to handle FAT and filesystems with perms

update local_copy_dir rsync to handle FAT and filesystems with perms

With FAT filesystems, the user, group, and permissions will not be at all
preserved.  With file systems like ext4 that have perms, the umask might
not be set to something that makes sense for the public repo files, which
are meant to be published and therefore readible by all.

If need be, it would be easy enough to add a config option for rsync's
chmod string, to address setups that have specific permissions needs.

fixes #23 https://gitlab.com/fdroid/fdroidserver/issues/23