Runes in Plan 9 were limited to the 16-bit BMP when I drew up the RPC protocol between graphical programs and devdraw a long time ago. Now that they can be 32-bit, use a 32-bit wire encoding too. A new message number to avoid problems with other clients (like 9fans.net/go). Add keyboard shortcut alt : , for U+1F602, face with tears of joy, to test that it all works.

Setting up for a real window system.

Fixes #340. Fixes #343.

Fixes #339.

Need to think a bit more about locking, but overall it's a working devdraw. Multiclient mode may not be working but nothing is using it yet.

Also turn mac-srv.c into a generic srv.c, so we can remove the duplication with x11-srv.c.