REDHAT-BUG-2436980

libssh can try to open any file during configuration parsing, when misconfigured or when local attacker can provide malicious configuration. This applies for all configuration loaded from default location, configuration provided through the `ssh_config_parse_file()` and `ssh_bind_config_parse_file()` functions as well as configuration files included from them directly or through glob wildcards. The possibly dangerous files involve block devices, fifo, named pipe or huge system files that could cause Denial of Service. The solution here is allowing to read only regular files and enforcing configuration file size limit of 16MB. Currently, maximum line length of a configuration file is 1K so this will effectively mean configuration files of 16K lines should still keep working.