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)