T171: Allow specifiying algorithms in configuration file by adding or removing algorithms from default

Description

Originally reported by ansasaki: https://bugs.libssh.org/T171

This is about adding support to adding or removing algorithms from default set in configuration files using the '+' and '-' characters. Then it would be possible to set algorithms like:

HostKeyAlgorithms -rsa-sha2-256

For the default set of algorithms except rsa-sha2-256

If possible, also support keywords like DEFAULT for the default set and FIPS for the fips allowed algorithms.