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_pthread_server.c

@@ -30,9 +30,13 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
+#include <stdlib.h>
 #define mbedtls_fprintf    fprintf
 #define mbedtls_printf     printf
 #define mbedtls_snprintf   snprintf
+#define mbedtls_exit            exit
+#define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
+#define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif
 
 #if !defined(MBEDTLS_BIGNUM_C) || !defined(MBEDTLS_CERTS_C) ||            \
@@ -77,6 +81,18 @@ int main( void )
 #include "mbedtls/memory_buffer_alloc.h"
 #endif
 
+#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 - %s\n",
+                    file, line, failure_condition );
+    mbedtls_exit( MBEDTLS_EXIT_FAILURE );
+}
+#endif
+
 #define HTTP_RESPONSE \
     "HTTP/1.0 200 OK\r\nContent-Type: text/html\r\n\r\n" \
     "<h2>mbed TLS Test Server</h2>\r\n" \