src: check hash update/final success

Also:
- delete unused internal macro `libssh2_md5()` where defined.
- prefix `libssh2_os400qc3_hash*()` function names with underscore.
  These are public/visible, but internal.
- add FIXMEs to OS/400 code to verify update/final calls; some OS API,
  some internal.

Ref: https://github.com/libssh2/libssh2/pull/1301#discussion_r1446861650
Reviewed-by: Michael Buckley
Reviewed-by: Patrick Monnerat
Closes #1303

include/libssh2.h

@@ -589,6 +589,7 @@ typedef struct _LIBSSH2_POLLFD {
 #define LIBSSH2_ERROR_ALGO_UNSUPPORTED          -51
 #define LIBSSH2_ERROR_MAC_FAILURE               -52
 #define LIBSSH2_ERROR_HASH_INIT                 -53
+#define LIBSSH2_ERROR_HASH_CALC                 -54
 
 /* this is a define to provide the old (<= 1.2.7) name */
 #define LIBSSH2_ERROR_BANNER_NONE LIBSSH2_ERROR_BANNER_RECV