Revert 295285 which was an MFC of the tryforward work (r290383,295282,295283)

In the IPFW+NAT+divergent MTU case there is a bug in sening ICMP MTU updates.

Approved by: re (marius, gjb)
Sponsored by: Rubicon Communications (Netgate)

MFC: r290383,295282,295283

Replace the fastforward path with tryforward which does not require a
sysctl and will always be on. The former split between default and
fast forwarding is removed by this commit while preserving the ability
to use all network stack features.

Differential Revision: https://reviews.freebsd.org/D4042
Reviewed by: ae, melifaro, olivier, rwatson
Approved by: re (glebius)
Sponsored by: Rubicon Communications (Netgate)

MFC r271545, 271610:
Make net.inet.ip.sourceroute, net.inet.ip.accept_sourceroute, and
net.inet.ip.process_options vnet-aware.

Merge r262763, r262767, r262771, r262806 from head:
- Remove rt_metrics_lite and simply put its members into rtentry.
- Use counter(9) for rt_pksent (former rt_rmx.rmx_pksent). This
removes another cache trashing ++ from packet forwarding path.
- Create zini/fini methods for the rtentry UMA zone. Via initialize
mutex and counter in them.
- Fix reporting of rmx_pksent to routing socket.
- Fix netstat(1) to report "Use" both in kvm(3) and sysctl(3) mode.

MFC r271545, 271610:
Make net.inet.ip.sourceroute, net.inet.ip.accept_sourceroute, and
net.inet.ip.process_options vnet-aware.

Merge r262763, r262767, r262771, r262806 from head:
- Remove rt_metrics_lite and simply put its members into rtentry.
- Use counter(9) for rt_pksent (former rt_rmx.rmx_pksent). This
removes another cache trashing ++ from packet forwarding path.
- Create zini/fini methods for the rtentry UMA zone. Via initialize
mutex and counter in them.
- Fix reporting of rmx_pksent to routing socket.
- Fix netstat(1) to report "Use" both in kvm(3) and sysctl(3) mode.