Sync OpenBSD patchset 968:

Try to resolve relative paths for loadb and saveb (first using client
working directory if any then default-path or session wd).

tmux.h

@@ -1341,6 +1341,7 @@ void		 logfile(const char *);
 const char	*getshell(void);
 int		 checkshell(const char *);
 int		 areshell(const char *);
+const char*	 get_full_path(const char *, const char *);
 void		 setblocking(int, int);
 __dead void	 shell_exec(const char *, const char *);