Implementation of strncasecmp() which returns the case-insensitive difference between two strings

To begin with, this is my first Pull Request. So sincere apologies if I missed anything.
 
This change applies to compilers that do not implement strncasecmp(). The modification implements this function and returns the difference between the first two non-matching characters in the strings. It also implements and uses upcase(), which is the equivalent of the toupper() function. This is target-specific and not built by default for the "native" target.