Add new guard `LIBSSH2_RSA_SHA1`. Add missing guards for `LIBSSH2_RSA`,
`LIBSSH2_DSA`.
Fix warnings when all options are disabled.
This is still not complete and it's possible to break a build with
certain crypto backends (e.g. mbedTLS) and/or combination of options.
It's not guaranteed that all bits everywhere get disabled by these
settings. Consider this a "best effort".
Add these new options to disable certain crypto elements:
- `LIBSSH2_NO_3DES`
- `LIBSSH2_NO_AES_CTR`
- `LIBSSH2_NO_BLOWFISH`
- `LIBSSH2_NO_CAST`
- `LIBSSH2_NO_ECDSA`
- `LIBSSH2_NO_RC4`
- `LIBSSH2_NO_RSA_SHA1`
- `LIBSSH2_NO_RSA`
The goal is to offer a way to disable legacy/obsolete/insecure ones.
See also: 146a25a06d `LIBSSH2_NO_HMAC_RIPEMD`
See also: 38015f4e46 `LIBSSH2_NO_DSA`
See also: be31457f30 `LIBSSH2_NO_MD5`
Closes#986
857e431648