move pfsync_state_import in if_pfsync.c to pf_state_import in pf.c

move pfsync_state_import in if_pfsync.c to pf_state_import in pf.c

this is straightening the deck chairs. the state import and export
code are used by both the pf ioctls and pfsync, but the export code
is in pf.c and the import code is in if_pfsync. if pfsync was
disabled then the ioctl stuff wouldnt link.

moving the import code to pf.c makes it more symmetrical(?) and
robust.

tweaks and ok from kn@ sashan@