Redo a bit of sendto logic, especially regarding flags
Change return type of command callbacks to `int'
4.3bsd has no `const', and fix stack corruption bug.

When depth is 0, p->indent[depth - {3,2}] writes outside the bounds
of the struct map_priv, which corrupts the u_user *u in m_map, causing
segfaults when sending RPL_MAPEND.