make if_pfsync.c a better friend with PF_LOCK

make if_pfsync.c a better friend with PF_LOCK

The code delivered in this change is currently disabled. Brave souls
may enable the code by adding -DWITH_PF_LOCK when building customized
kernel. Big thanks goes to Hrvoje@ for providing test equipment and
testing.

As soon as we enter the next release cycle, the WITH_PF_LOCK will be
defined as default option for MP kernels.

OK dlg@