crypto: add/fix algo guards and extend NO options

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

src/openssl.h

@@ -105,9 +105,11 @@
 
 #ifdef OPENSSL_NO_RSA
 # define LIBSSH2_RSA 0
+# define LIBSSH2_RSA_SHA1 0
 # define LIBSSH2_RSA_SHA2 0
 #else
 # define LIBSSH2_RSA 1
+# define LIBSSH2_RSA_SHA1 1
 # define LIBSSH2_RSA_SHA2 1
 #endif
 
@@ -353,13 +355,17 @@ extern void _libssh2_openssl_crypto_exit(void);
 #define libssh2_crypto_init() _libssh2_openssl_crypto_init()
 #define libssh2_crypto_exit() _libssh2_openssl_crypto_exit()
 
+#if LIBSSH2_RSA
 #define libssh2_rsa_ctx RSA
 
 #define _libssh2_rsa_free(rsactx) RSA_free(rsactx)
+#endif
 
+#if LIBSSH2_DSA
 #define libssh2_dsa_ctx DSA
 
 #define _libssh2_dsa_free(dsactx) DSA_free(dsactx)
+#endif
 
 #if LIBSSH2_ECDSA
 #define libssh2_ecdsa_ctx EC_KEY