merge_flags
serial.g_iflags = merge_flags(sp->g_iflags, pmptr->p_ttymode);
serial.g_fflags = merge_flags(sp->g_fflags, pmptr->p_ttymode);
int merge_flags = INSERT_ONLY;
merge_flags |= COALESCE_LEFT;
merge_flags |= COALESCE_RIGHT;
switch (merge_flags) {