md5: allow disabling old-style encrypted private keys at build-time

Before this patch, this happened at runtime when using an old (pre-3.0),
FIPS-enabled OpenSSL backend.

This patch makes it possible to disable this via the build-time option
`LIBSSH2_NO_MD5_PEM`.

Also:
- make sure to exclude all MD5 internal APIs when both the above and
  `LIBSSH2_NO_MD5` are enabled.
- fix tests to support build with`LIBSSH2_NO_MD5`, `LIBSSH2_NO_MD5_PEM`
  and `LIBSSH2_NO_3DES`.
- add FIXME to apply this change to `os400qc3.*`.

Old-style encrypted private keys require MD5 and they look like this:
```
-----BEGIN RSA PRIVATE KEY-----
 Proc-Type: 4,ENCRYPTED
 DEK-Info: AES-128-CBC,<MD5-hex>

 <base64>
 -----END RSA PRIVATE KEY-----
```

E.g.: `tests/key_rsa_encrypted`

Ref: https://github.com/libssh2/www/issues/20
Closes #1181

tests/test_auth_pubkey_ok_rsa_encrypted.c

@@ -7,7 +7,7 @@
 
 int test(LIBSSH2_SESSION *session)
 {
-#if LIBSSH2_RSA_SHA1
+#if LIBSSH2_RSA_SHA1 && LIBSSH2_MD5_PEM
     /* set in Dockerfile */
     return test_auth_pubkey(session, 0,
                             "libssh2",