Break the common process set up, event loop and imsg dispatch code

between server and client out into a separate internal API. This will
make it easier to add another process.

tmux.c

@@ -29,6 +29,7 @@
 #include <pwd.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 #include <unistd.h>
 
 #include "tmux.h"
@@ -347,9 +348,6 @@ main(int argc, char **argv)
 	}
 	free(path);
 
-	/* Set process title. */
-	setproctitle("%s (%s)", __progname, socket_path);
-
 	/* Pass control to the client. */
 	exit(client_main(event_init(), argc, argv, flags));
 }