Change message modifying operations to additively set redraw flags.

Change message modifying operations to additively set redraw flags.

With the ability to set redraw flags with the menu stack operations,
some operations internally modify the current menu redraw flag.
For instance, _mutt_set_flag() can now set REDRAW_SIDEBAR.

Change the ops that modify messages to use 'redraw |= REDRAW_X'
instead of overwriting the flag value.