Move all calls to fcntl(...O_NONBLOCK) into a function and clear the

flag on the stdio file descriptors before closing them (fixes things
like "tmux ls && cat").

tmux.h

@@ -1308,6 +1308,7 @@ void		 logfile(const char *);
 const char	*getshell(void);
 int		 checkshell(const char *);
 int		 areshell(const char *);
+void		 setblocking(int, int);
 __dead void	 shell_exec(const char *, const char *);
 
 /* cfg.c */