Run two-stages backups (#1)

1. Download to a local directory, archive/encrypt there
1. Push encrypted archive to remote folder

This helps when the backup destination is e.g. a NFS drive.

Also, introduce harder checks and fix some flaws.

Reviewed-on: #1
Co-authored-by: Max Mehl <mail@mehl.mx>
Co-committed-by: Max Mehl <mail@mehl.mx>

README.md

@@ -63,6 +63,21 @@ shortcuts:
 You can give multiple locations that shall be backed up. Just separate them by
 `|` characters. See the example file for more.
 
+## Process
+
+The script runs the following most important steps:
+1. For each host in `hosts.csv`
+    1. Check SSH connection
+    1. Compose SSH host settings
+    1. For each backup source
+        1. Resolve special backup sources
+        1. Create backup destination
+        1. rsync source to destination
+        1. tar the destination
+        1. gpg-encrypt the destination
+        1. Delete older backups
+1. Output completion info
+
 ## Manual run
 
 You can run `ssh-checker.sh` and `uberspace-backup.sh` manually. Without any arguments given, both will check/backup all hosts.