Fix const-ness in mbedtls_param_failed()

The previous prototype gave warnings are the strings produced by #cond and
__FILE__ are const, so we shouldn't implicitly cast them to non-const.

While at it modifying most example programs:
- include the header that has the function declaration, so that the definition
  can be checked to match by the compiler
- fix whitespace
- make it work even if PLATFORM_C is not defined:
    - CHECK_PARAMS is not documented as depending on PLATFORM_C and there is
      no reason why it should
    - so, remove the corresponding #if defined in each program...
    - and add missing #defines for mbedtls_exit when needed

The result has been tested (make all test with -Werror) with the following
configurations:

- full with    CHECK_PARAMS with    PLATFORM_C
- full with    CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS with    PLATFORM_C

Additionally, it has been manually tested that adding

    mbedtls_aes_init( NULL );

near the normal call to mbedtls_aes_init() in programs/aes/aescrypt2.c has the
expected effect when running the program.

programs/ssl/mini_client.c

@@ -26,6 +26,17 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
+#if defined(MBEDTLS_PLATFORM_C)
+#include "mbedtls/platform.h"
+#else
+#include <stdio.h>
+#include <stdlib.h>
+#define mbedtls_printf          printf
+#define mbedtls_exit            exit
+#define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
+#define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
+#endif
+
 /*
  * We're creating and connecting the socket "manually" rather than using the
  * NET module, in order to avoid the overhead of getaddrinfo() which tends to
@@ -44,13 +55,6 @@
     !defined(MBEDTLS_NET_C) || !defined(MBEDTLS_SSL_CLI_C) || \
     !defined(UNIX)
 
-#if defined(MBEDTLS_PLATFORM_C)
-#include "mbedtls/platform.h"
-#else
-#include <stdio.h>
-#define mbedtls_printf printf
-#endif
-
 int main( void )
 {
     mbedtls_printf( "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_ENTROPY_C and/or "
@@ -60,12 +64,6 @@ int main( void )
 }
 #else
 
-#if defined(MBEDTLS_PLATFORM_C)
-#include "mbedtls/platform.h"
-#else
-#include <stdlib.h>
-#endif
-
 #include <string.h>
 
 #include "mbedtls/net_sockets.h"
@@ -168,10 +166,14 @@ enum exit_codes
     ssl_write_failed,
 };
 
-#if defined( MBEDTLS_CHECK_PARAMS ) && defined(MBEDTLS_PLATFORM_C)
-void mbedtls_param_failed( char* failure_condition, char* file, int line )
+#if defined(MBEDTLS_CHECK_PARAMS)
+#include "mbedtls/platform_util.h"
+void mbedtls_param_failed( const char *failure_condition,
+                           const char *file,
+                           int line )
 {
-    mbedtls_printf("%s:%i: Input param failed - %sn", file, line,                           failure_condition );
+    mbedtls_printf( "%s:%i: Input param failed - %s\n",
+                    file, line, failure_condition );
     mbedtls_exit( MBEDTLS_EXIT_FAILURE );
 }
 #endif