Add sketch code to reassemble a DHCP packet from our internal "everything

is a DHCP option" data structures.

We need this code in order to be able to return a DHCP packet to a PXE NBP
which reflects options from our multiple sources (e.g. NVS and DHCP
server).  This is expensive, but necessary.  Having paid this cost, we may
as well try to use the same code to generate our DHCP request packets,
since the process is similar.

src/include/errno.h

@@ -151,6 +151,7 @@
 #define ENOENT		0xe8	/**< No such file or directory */
 #define ENOEXEC		0xe9	/**< Exec format error */
 #define ENOMSG		ENODATA	/**< No message of the desired type */
+#define ENOSPC		ENOMEM	/**< No space left on device */
 #define ENOSR		0xea	/**< No stream resources */
 #define ENOSTR		0xeb	/**< Not a stream */
 #define ENOSYS		0xec	/**< Function not implemented */