build: enable -Wcast-qual, fix fallouts

- enable compiler warning `-Wcast-qual`.
- add `LIBSSH2_UNCONST()` macro to strip const where absolutely
  necessary to avoid compiler warnings.
- fix const stripping by constifying where necessary.
- fix const stripping by using `LIBSSH2_UNCONST()`.
- libgcrypt.h: drop unnecessary casts.
- openssl: fix to use new `BIO_new_mem_buf()` parameter types
  with wolfSSL.

Cherry-picked from #1484
Closes #1527

example/ssh2_agent_forwarding.c

@@ -84,7 +84,7 @@ int main(int argc, char *argv[])
     LIBSSH2_AGENT *agent = NULL;
     struct libssh2_agent_publickey *identity, *prev_identity = NULL;
     int exitcode;
-    char *exitsignal = (char *)"none";
+    char *exitsignal = NULL;
     ssize_t bytecount = 0;
 
 #ifdef _WIN32
@@ -272,7 +272,8 @@ int main(int argc, char *argv[])
     }
 
     if(exitsignal) {
-        fprintf(stderr, "\nGot signal: %s\n", exitsignal);
+        fprintf(stderr, "\nGot signal: %s\n",
+                exitsignal ? exitsignal : "none");
     }
     else {
         fprintf(stderr, "\nEXIT: %d bytecount: %ld\n",