Algorithms are comma separated.
Note that valid values does not in all cases represent default values.
`--sftp=pub-key-algos=...` specifies the supported client public key
authentication algorithms. Note that this doesn't include certificate types
since those use the underlying algorithm. This list is sent to the client if
it supports the server-sig-algs extension. Order is irrelevant.
Valid values
```
ssh-ed25519
sk-ssh-ed25519@openssh.comsk-ecdsa-sha2-nistp256@openssh.com
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
rsa-sha2-256
rsa-sha2-512
ssh-rsa
ssh-dss
```
`--sftp=kex-algos=...` specifies the supported key-exchange algorithms in preference order.
Valid values:
```
curve25519-sha256
curve25519-sha256@libssh.org
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group14-sha1
diffie-hellman-group1-sha1
```
`--sftp=cipher-algos=...` specifies the allowed cipher algorithms.
If unspecified then a sensible default is used.
Valid values:
```
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.comaes256-gcm@openssh.comchacha20-poly1305@openssh.com
arcfour256
arcfour128
arcfour
aes128-cbc
3des-cbc
```
`--sftp=mac-algos=...` specifies a default set of MAC algorithms in preference order.
This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed because they have
reached the end of their useful life.
Valid values:
```
hmac-sha2-256-etm@openssh.comhmac-sha2-512-etm@openssh.com
hmac-sha2-256
hmac-sha2-512
hmac-sha1
hmac-sha1-96
```
Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.
Also introduce bugLogIf() for errors that should not happen in the first
place.
