Add chapter 18 exercises (still missing custom nftw)

@rmscarenhas  :disappointed: 
Apply this patch and run the program again to see what I mean :smile: 

~~~patch
diff --git a/c18/getcwd.c b/c18/getcwd.c
index 6e44b7e..698ccfb 100644
--- a/c18/getcwd.c
+++ b/c18/getcwd.c
@@ -32,6 +32,7 @@ char * _getcwd(char *, size_t);
 
 int main(int argc, char *argv[]) {
   char buffer[path_max];
+  snprintf(buffer, path_max, "zomg barbecue");
 
   if (_getcwd(buffer, path_max) == NULL) {
     pexit("_getcwd");
~~~