System tablespace can be specified with absolute path, if innodb_data_home_dir
is empty.

This was not handled well  by mariabackup

1. In backup phase, empty innodb_data_home_dir variable from server was
not recognized. full paths were stored in backup-my.ini, even if
it stored all files locally. thus prepare phase would not find the system
tablespace files.

2. In copy-back phase, copy would not be done to the absolute destination
path, as path would be stripped with trim_dotslash

This patch fixes the above defects.