Call setbuf when reading or writing files: library

After opening a file containing sensitive data, call mbedtls_setbuf() to
disable buffering. This way, we don't expose sensitive data to a memory
disclosure vulnerability in a buffer outside our control.

This commit adds a call to mbedtls_setbuf() after each call to fopen(),
except:
* In ctr_drbg.c, in load_file(), because this is only used for DH parameters
  and they are not confidential data.
* In psa_its_file.c, in psa_its_remove(), because the file is only opened
  to check its existence, we don't read data from it.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/dhm.c

@@ -620,6 +620,7 @@ static int load_file( const char *path, unsigned char **buf, size_t *n )
 
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_DHM_FILE_IO_ERROR );
+    /* The data loaded here is public, so don't bother disabling buffering. */
 
     fseek( f, 0, SEEK_END );
     if( ( size = ftell( f ) ) == -1 )