Assorted code cleanups
- Clear
-Wunused-macroswarnings whenarc4random_buf()is available -
XdmcpWriteHeader: drop unnecessary cast offree()argument -
XdmcpWriteHeader: usecallocinstead ofmallocto alloc buffer
-Wunused-macros warnings when arc4random_buf() is availableXdmcpWriteHeader: drop unnecessary cast of free() argumentXdmcpWriteHeader: use calloc instead of malloc to alloc buffer