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/ssl_fork_server.c

@@ -33,6 +33,7 @@
 #define mbedtls_fprintf         fprintf
 #define mbedtls_printf          printf
 #define mbedtls_time_t          time_t
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -86,10 +87,14 @@ int main( void )
 
 #define DEBUG_LEVEL 0
 
-#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