A pointer to the first occurence of c in str or NULL if c is not
found in str.
NOTES
EXAMPLE
char buffer[64];
strcpy (buffer, "Hello ");
// This returns a pointer to the second l in buffer.
strrchr (buffer, 'l');
// This returns NULL
strrchr (buffer, 'x');
It might seem that the algorithm below is slower than one which
first finds the end and then walks backwards but that would mean
to process some characters twice - if the string doesn't contain
c, it would mean to process every character twice.