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.