libc: Implement setbuf(), setbuffer(), and setlinebuf()

These are all simple wrappers around setvbuf().

libc/include/stdio.h

@@ -132,6 +132,8 @@ extern "C"
     /* Clear the error and end-of-file indicators in stream. */
     void clearerr(FILE* stream);
 
+    void setbuf(FILE*, char*);
+
     /* Write formatted output to a file. */
     int fprintf(FILE* stream, const char* format, ...);