When sending SIGUSR1/SIGUSR2 to a server process, all currently
running client instances change their theme. But before this patch,
all future instances used the original theme. With this patch, the
server owned config object is updated with the selected theme, thus
making new instances use the same theme as well.
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.
