Proerply virtualize pfsync for bringup after pf is initialized and
teardown of VNETs once pf(4) has been shut down.
Properly split resources into VNET_SYS(UN)INITs and one time module
loading.
While here cover the INET parts in the uninit callpath with proper
#ifdefs.

Approved by: re (gjb)
Obtained from: projects/vnet
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation