for layer 3 protocols that does not support path MTU discovery

for layer 3 protocols that does not support path MTU discovery
(I mean, IPv4) do not try to use rmx_mtu on routing table.
this symptom was introduced by rmx_mtu initialization (necessary for IPv6
path MTU discovery) in net/route.c.  now prior behavior is recovered.
From: Hugh Graham <hugh@openbsd.org>

there are several question about mssdflt semantics, though:

Question 1: with the current code, mssdflt does not override rmx_mtu value
(mssdflt overrides interface mtu only).  should we override rmx_mtu by
mssdflt as well?

Question 2: with the current code, mssdflt overrides mss computed from
if mtu, only when the destination is IPv4 non-local.  is it safe enough?
we may want to use mssdflt, whenever we are uncertain.
mss = if mtu - hdrsiz;
if (IPv4 non-local destination)
	mss = min(mss, mssdflt);