Replace sprintf with snprintf (#802)

Replace sprintf with snprintf (#802)

This change necessitates the introduction of 2 new APIs:

void vTaskListTasks( char * pcWriteBuffer, size_t uxBufferLength );
void vTaskGetRunTimeStatistics( char * pcWriteBuffer, size_t uxBufferLength );

These 2 APIs behave exactly as vTaskList and vTaskGetRunTimeStats
except the fact that they take the length of the pcWriteBuffer as the
second argument to ensure that we do not write past the buffer.

vTaskList and vTaskGetRunTimeStats assume that the length of the
buffer is configSTATS_BUFFER_MAX_LENGTH which defaults to 0xFFFF.
This is done to ensure that the existing applications do not break.
New applications should use the new APIs to avoid memory corruption.