config: Include relative files
The ssh config supports relative include directive:
Files without absolute paths are assumed to be in ~/.ssh if included in a user configuration file or /etc/ssh if included from the system configuration file.
The libssh currently accepts only absolute paths, which makes it confusing for users using this corner case such as in Remmina/Remmina#2604 (closed).
Libssh should try to support also this corner case to provide less surprises for users using libssh in other applications.