server: implement a --server mode

In this mode, foot listens on a UNIX socket and creates terminal
windows when clients connect.

A connecting client sends argc/argv to the server, and the server
instantiates a new terminal window.

When the terminal window is closed, the exit code is sent back to the
client.

meson.build

@@ -78,6 +78,7 @@ executable(
   'render.c', 'render.h',
   'search.c', 'search.h',
   'selection.c', 'selection.h',
+  'server.c', 'server.h',
   'shm.c', 'shm.h',
   'slave.c', 'slave.h',
   'terminal.c', 'terminal.h',