Add a "back to indentation" key in copy mode to move the cursor to the first

non-whitespace character. ^ with vi and M-m with emacs key bindings. Another
from Kalle Olavi Niemitalo, thanks.

status.c

@@ -709,6 +709,7 @@ status_prompt_key(struct client *c, int key)
 		}
 		break;
 	case MODEKEYCMD_STARTOFLINE:
+	case MODEKEYCMD_BACKTOINDENTATION:
 		if (c->prompt_index != 0) {
 			c->prompt_index = 0;
 			c->flags |= CLIENT_STATUS;