Resolve the remote output path on the server

The output path was made absolute with os.path.abspath and '~' was
expanded to the local home, so relative paths and '~' pointed to the
wrong folder on the server. Resolve the path on the server instead,
where '~' and relative paths refer to the remote user's home, and
document it.

simple_backup/simple_backup.conf

@@ -4,7 +4,7 @@
 # Files and directories to backup. Multiple items can be separated using a comma (','). It is possible to use wildcards (i.e. '*' to match multiple characters and '~' for the user's home directory).
 inputs=/home/user
 
-# Output directory.
+# Output directory. For remote backups, this is a path on the server ('~' and relative paths refer to the home directory on the server).
 backup_dir=/media/Backup
 
 # Files, directories and patterns to exclude from the backup. Multiple items can be separated using a comma.