spawn login shells by default, adapt manpage bits as well.

ok nicm@

tmux.1

@@ -947,7 +947,7 @@ Set the command used for new windows (if not specified when the window is
 created) to
 .Ar command .
 The default is
-.Dq exec $SHELL .
+.Dq exec $SHELL -l .
 .It Ic default-path Ar path
 Set the default working directory for processes created from keys, or
 interactively from the prompt.

tmux.c

@@ -369,7 +369,7 @@ main(int argc, char **argv)
 			shell = _PATH_BSHELL;
 	}
 	options_set_string(
-	    &global_options, "default-command", "exec %s", shell);
+	    &global_options, "default-command", "exec %s -l", shell);
 
 	if (getcwd(cwd, sizeof cwd) == NULL) {
 		log_warn("getcwd");